Improves PHP-CS-Fixer workflow

[Sesquipedalian]

1. As requested by @jdarwood007, now pegs the call to tj-actions/changed-files to a specific hash rather than a version. Note that when I tried reverting to the raw shell code that was used prior to Fixes PHP-CS-Fixer workflow  #9110, the action stopped working, which strongly suggests that said shell code was the cause of the problem that Fixes PHP-CS-Fixer workflow  #9110 fixed.
2. To address an issue raised by @Oldiesmann, the error message in the action output now shows the changes that need to be made in order to comply with our code standard.