Permit GitHub Actions CI to write Packages, needed for php-build caching

[osma]

Reasons for creating this PR

I'm trying to get tests on GitHub Actions CI working again. This PR fixes part of the problem - the permission issue preventing php-build from caching PHP containers as Packages.

Link to relevant issue(s), if any

• Related to GitHub Actions CI tests are failing on PHP 7.x #1288 but doesn't fix everything

Description of the changes in this PR

Allow Actions to write Packages according to GitHub documentation

Known problems or uncertainties in this PR

Tests are still broken on PHP 7.x (see #1288) but at least the PHP containers get built and stored as Packages and tests on PHP 8.0 are passing.

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if not, explain why below)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[sonarcloud]

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