Allow explicitly setting environment file location

This is a possible solution to #3315

`SS_ENVIRONMENT_FILE` can now be defined via an environment variable with the same name OR via a CLI option

```
framework/sake url --environment-file=/path/to/_ss_environment.php
```

This also BREAKS the CLI behaviour where `--` prefixed options would be parsed as a URL query string:

```
framework/sake url --flush=all&something=somethingelse
```

Instead, we'd have to use the current behaviour:

```
framework/sake url flush=all something=somethingelse
```
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