GH Actions: fix build

[jrfnl]

Both Composer, as well as PHPUnit have released new versions to try and fix the issue with tests being run with process isolation in combination with installation via Composer 2.2, which were resulting in the following error:

PHPUnit\Framework\Exception: PHP Fatal error:  strict_types declaration must be the very first statement in the script in /home/runner/work/BrainMonkey/BrainMonkey/vendor/phpunit/phpunit/phpunit on line 2

While that fixes the builds for runs against the highest versions of dependencies, it still does not solve the problem when running the tests with the lowest dependencies (or against PHP 5.6).

Those builds now hang indefinitely.

This commit attempts to fix this by using Composer 2.1 for builds against lowest dependencies and against PHP 5.6.

It also sets a 15 minute time-out for the test runs against the lowest dependencies to prevent builds being very slow in reporting.

Refs:

• https://github.com/composer/composer/issues/10387
• https://github.com/sebastianbergmann/phpunit/pull/4846
• https://github.com/composer/composer/releases/tag/2.2.2
• https://github.com/sebastianbergmann/phpunit/blob/9.5/ChangeLog-9.5.md#9511---2021-12-25

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Note: I will rebase the other open PRs after this PR has been merged to get their builds passing.

[jrfnl]

Some interesting additional information about this issue in the follow-up conversation here: https://github.com/composer/composer/issues/10387#issuecomment-1002704203

[jrfnl]

@gmazzap Looks like there may be a more comprehensive fix coming, so let's hold off on merging this one for now. I'm going to test the more comprehensive fix now.

[jrfnl]

With the latest release of Composer (2.2.3), this PR is no longer necessary. The build for master is now passing again.

Closing this PR and I will be rebasing the other open PR to get passing builds.