check-files workflow: Count script/ as TR-side changes

[kfranqueiro]

This fixes a bug in the "isolation of changes to TR space or informative docs" check, which only considered guidelines to be TR space, disregarding scripts where wcag.js is stored.

Testing with the branch for #4590...

Old commands:

$ git diff main --numstat | grep -c '\sguidelines'
1
$ git diff main --numstat | grep -vc '\sguidelines'
1

New commands:

$ git diff main --numstat | egrep -c '\sguidelines/|\sscript/'
2
$ git diff main --numstat | egrep -vc '\sguidelines/|\sscript/'
0

(The check fails when the number is nonzero on both sides.)

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