Restrict the linting workflow to pushes/prs with php files

[IanDelMar]

Types of changes

Fix for the build process.

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• (Optional) My change requires a change to the documentation.
• (Optional) I have updated the documentation accordingly.
• (Optional) My change requires a change to the translations.
• (Optional) I have updated the translations accordingly.
• composer cs:check has passed locally.
• composer lint:php has passed locally.
• I have read the CONTRIBUTING document.

Related Issues or Roadmap requests

Further comments