fix issue where $_ENV contains array vals

There are cases where $env or $_ENV can contain a value that is an array
This will cause Process to throw an Array to String conversion exception
Initially I submitted a patch of Process.php, however Fabien indicated
that it shouldn't be fixed there (see below pull request).

Before recently, a simple work around would be in php.ini to set:

  register_argc_argv = On

However with recent changes, this seems to no longer work.

Original pull request: https://github.com/symfony/symfony/pull/7354

See ticket https://github.com/symfony/symfony/issues/7196
merged branch oodle/ticket_7196 (PR #8227)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #8227).

Discussion
----------

[Process] fix issue where $_ENV contains array vals

There are cases where $env or $_ENV can contain a value that is an array
This will cause Process to throw an Array to String conversion exception
Initially I submitted a patch of Process.php, however Fabien indicated
that it shouldn't be fixed there (see below pull request).

Before recently, a simple work around would be in php.ini to set:

  register_argc_argv = On

However with recent changes, this seems to no longer work.

Original pull request: https://github.com/symfony/symfony/pull/7354

See ticket https://github.com/symfony/symfony/issues/7196

```
| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 7196
| License       | MIT
| Doc PR        | none
```

Commits
-------

2c6af95 [Process] fix issue where $_ENV contains array vals
[Process] moved env check to the Process class (refs #8227)