CI/CD: Use private cache of (third-party) GitHub Actions.

[briansmith]

This is a step towards implementing the GitHub security hardening advice.

sed -ri 's|(uses: +)([^/]+)/([^@])|\1briansmith/\2-\3|g' .github/workflows/ci.yml

[codecov]

Codecov Report

Merging #52 (4a39cba) into main (6a15d6e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #52   +/-   ##
=======================================
  Coverage   75.72%   75.72%           
=======================================
  Files           5        5           
  Lines         206      206           
=======================================
  Hits          156      156           
  Misses         50       50           

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[briansmith]

After the first run of this passed, I changed the Github Actions permissions for this repo to "Allow local actions only."

[briansmith]

$ grep "uses:" .github/workflows/ci.yml | sort | uniq
      - uses: briansmith/actions-cache@v2
      - uses: briansmith/actions-checkout@v2
      - uses: briansmith/actions-rs-toolchain@v1
      - uses: briansmith/codecov-codecov-action@v1