ci: support STEP_DEBUG in Code Quality workflow

[justlevine]

What does this implement/fix? Explain your changes.

This PR adds support for using STEP_DEBUG in the Code Quality workflow to run phpstan in debug mode.

This change will allow better debugging of CI failures that cannot be easily replicated locally.
For example: if hypothetically a leaky dependency causes phpstan to throw an Out of Memory issues, this would let us pinpoint that the error isnt occurring due to the contents of the PR

To test

1. Visit the Code Quality action on this PR
2. Click 'Rerun All Jobs'
3. Check Enable debug logging and click Re-run jobs.
4. Check log for Run PHPStan to see the PHPStan messages output by the --debug flag.

Does this close any currently open issues?

…

Any relevant logs, error output, GraphiQL screenshots, etc?

• Step Debugging Enabled: https://github.com/wp-graphql/wp-graphql/actions/runs/5416920029/jobs/9847278392?pr=2841#step:6:55

Any other comments?

Where has this been tested?

Operating System: GitHub Actions

WordPress Version: N/A

[codeclimate]

Code Climate has analyzed commit a87c02f and detected 0 issues on this pull request.

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[coveralls]

coverage: 85.064%. remained the same when pulling a87c02f on justlevine:ci/phpstan-step-debug into 9f4eb31 on wp-graphql:develop.