[CI] Fix diff regex for JS Mocha templates

Summary: The runOnDiffRegex value doesn't expand the environment variables (and shouldn't) causing the match to fail.

Test Plan: See the diffs like D13634 stopped running on CI after the regex moved to the template.

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13676