ci: Update test-wpcom-filename-restrictions

[anomiex]

Closes MONOREP-176

Proposed changes:

Since 147-ghe-Automattic/pre-receive-hooks, 130_stop_executables runs on modified files too, but only prevents changing them to be executable (not existing executables).

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

None

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Try making a PR based on this one that changes an existing file (that makes it into Jetpack or jetpack-mu-wpcom-plugin) executable. See if the appropriate Post-Build check fails.

[github-actions]

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[tbradsha]

Good catch.