Command `dbt:run` should work without initializing extra environment variables

When running `meltano elt tap target --transform=run` - things "just work" without little additional work if you're using one of the existing transform packages like in the gitlab demo project in a pipeline.

However, running `meltano invoke dbt:run` doesn't work because there are still several env vars we have to set. The big ones (like DBT_TARGET_SCHEMA) are covered very briefly in https://meltano.com/docs/transforms.html#running-a-transform-in-meltano but there's a lot more you need to potentially set if you're trying to use it from the env, namely: PG_DATABASE|PASSWORD|USERNAME|PORT|etc.

At first, I couldn't tell if those were all env vars we were just setting proactively every time you run `meltano elt` (because we can infer what we need there and what the target context is), or if there was another way I should configure this. i.e. is there a generated config file I should just reference? All the config files I found just seem to reference env vars though.

## Environment Variables currently expected by our bundled `profiles.yml`

| Variable            | Meaning                                                                                                                                        | Maps to                                                                                                                                                                                                                                                                                                                                  |
| ------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `DBT_TARGET`        | > This is the default target your dbt project will use. It must be one of the targets you define in your profile. Commonly it is set to `dev`. | Defaults to Meltano's `$MELTANO_LOAD__DIALECT`. With the `profiles.yml` currently packaged in the dbt file bundle, this means one of `snowflake`, `postgres` or `bigquery`. Like @tayloramurphy mentions in https://gitlab.com/meltano/meltano/-/issues/3104#note_804770702, this is confusing and not standard practice in dbt projects. |
| `DBT_TARGET_SCHEMA` | > The default schema that dbt will build objects in.                                                                                           | Defaults to `analytics`.                                                                                                                                                                                                                                                                                                                 |
| `SF_*`              | Snowflake connection parameters.                                                                                                               | These [leverage the `env_aliases`](https://gitlab.com/meltano/meltano/-/blob/544f7128b3efd783efe7ac7b7c109e80ef9f086c/src/meltano/core/bundle/discovery.yml#L2551-2578) injected at runtime by Meltano. So, they get the same values as `TARGET_SNOWFLAKE_*`.                                                                                                                                                                                                                   |
| `SF_ROLE`           | Snowflake Role.                                                                                                                                | This one doesn't use `env_aliases` so I'm not sure if it gets injected...                                                                                                                                                                                                                                                                |
| `PG_*`              | PostgreSQL connection parameters.                                                                                                              | These [leverage the `env_aliases`](https://gitlab.com/meltano/meltano/-/blob/544f7128b3efd783efe7ac7b7c109e80ef9f086c/src/meltano/core/bundle/discovery.yml#L2299-2333) injected at runtime by Meltano. So, they get the same values as `TARGET_POSTGRES_*`.                                                                                                                                                                                                                    |
| `TARGET_BIGQUERY_*` | BigQuery connection parameters.                                                                                                                | These use directly the same values as `TARGET_BIGQUERY_*`.                                                                                                                                                                                                                                                                               |

Story points: 4.0