[GHA] Don't run front/back-end job on purely back/front-end pr

[imnasnainaec]

The backend docker build and frontend test coverage checks are two of our longest-running, so it's nice to skip them when they're completely irrelevant to the pr at hand.

Only adding the folders/files that are most frequently changed in isolation and that will always be irrelevant to the workflow. (Trying to come up with a comprehensive inclusion list instead would create a maintenance nightmare.)

This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.64%. Comparing base (9734714) to head (e6c0426).
⚠️ Report is 26 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4035   +/-   ##
=======================================
  Coverage   74.64%   74.64%           
=======================================
  Files         293      293           
  Lines       10829    10829           
  Branches     1355     1355           
=======================================
  Hits         8083     8083           
  Misses       2351     2351           
  Partials      395      395           

Flag	Coverage Δ
backend	85.55% <ø> (ø)
frontend	66.23% <ø> (ø)

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[jasonleenaylor]

@jasonleenaylor reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)