chore: Cache custom golangci-lint binaries in GHA workflow

[alexandear]

This speeds up the linter job from 3m30s to 2m30s in GitHub Actions when the cache hits.

Updates #4070

Changes:

• Cache golangci-lint and custom-gcl in GitHub Actions.
• Rename extraneous-new to extraneousnew for consistency with other custom linters.
• Derive the golangci-lint version in setup-custom-gcl.sh from .custom-gcl.yml. .custom-gcl.yml is a single source of truth for the golangci-lint version.

[gmlewis]

I'm very sad that you are removing my beautiful hyphen, but otherwise, LGTM.
Thank you, @alexandear!
Merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (19c1a8a) to head (8ac904a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4116   +/-   ##
=======================================
  Coverage   93.68%   93.68%           
=======================================
  Files         211      211           
  Lines       19510    19510           
=======================================
  Hits        18278    18278           
  Misses       1034     1034           
  Partials      198      198           

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