CI/CD: Use my own fork 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
$ 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

[codecov]

Codecov Report

Merging #1257 (c821aac) into main (f3d8b52) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1257      +/-   ##
==========================================
+ Coverage   92.86%   92.87%   +0.01%     
==========================================
  Files         116      116              
  Lines       18270    18270              
==========================================
+ Hits        16967    16969       +2     
+ Misses       1303     1301       -2     

Impacted Files	Coverage Δ
src/aead/aes.rs	77.43% <0.00%> (-0.45%)	⬇️
crypto/cpu-intel.c	84.70% <0.00%> (+3.52%)	⬆️

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[briansmith]

After merging this, I just changed the Allowed Actions setting of this repository to "Allow local actions only."