Run pint in check mode for pull requests from forks

[sadiqk2]

php-code-styling fails on every pull request that comes from a fork, including #127, #128 and #136. It is not a code style problem, the job dies before Pint even runs:

[command]/usr/bin/git branch --list --remote origin/fix/security-cookie-config-namespace
[command]/usr/bin/git tag --list fix/security-cookie-config-namespace
##[error]A branch or tag with the name 'fix/security-cookie-config-namespace' could not be found

Two things are going on.

1. The checkout passes ref: github.head_ref but no repository:, so it looks for the fork's branch inside this repository and cannot find it. build-plugin.yml already passes repository: ${{ github.event.pull_request.head.repo.full_name }}, this workflow was never given the same treatment.

2. Even with the checkout fixed, the auto fix path cannot work for a fork. GITHUB_TOKEN has no write access to the contributor's fork, so git-auto-commit-action gets a 403 and the job fails again with a confusing error instead of a code style report.

This change:

• adds repository: to the checkout, so the fork's branch is actually fetched
• runs Pint in testMode for fork pull requests, so the job reports real style problems and passes when there are none
• keeps the existing auto fix and commit behaviour for pull requests from branches in this repository

Behaviour for pushes and for internal branches is unchanged.