…le (acasademont)

This PR was merged into the 5.3-dev branch.

Discussion
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[PhpUnitBridge] Add SYMFONY_PHPUNIT_REQUIRE env variable

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #39387
| License       | MIT
| Doc PR        | symfony/symfony-docs#14913

As discussed on #39387, this PR adds a new `SYMFONY_PHPUNIT_REQUIRE` env variable to add packages to the phpunit bridge installation. This is useful for adding phpunit plugins (ie: https://github.com/phpspec/prophecy-phpunit) without having to add them directly to the main app composer.json.

On my `phpunit.xml.dist` file I can now add

```xml
<server name="SYMFONY_PHPUNIT_REQUIRE" value="phpspec/prophecy-phpunit"/>
```

And the `phpspec/prophecy-phpunit` will be installed along the rest of the phpunit packages

Commits
-------

94e1d877eb Add SYMFONY_PHPUNIT_REQUIRE env variable Fixes #39387