chore: fix CI to use newer npm on node 14

[aabmass]

The npm version coming with Node 14 in Github Actions is v6 which does not support lockfile v3 checked into this repo:

npm WARN read-shrinkwrap This version of npm is compatible with lockfileVersion@1, but package-lock.json was generated for lockfileVersion@3. I'll try to do my best with it!

which results in non-hermetic build (see #297). This PR upgrades npm to the newest version supported in Node 14, which understands the lockfile to pull in exact dependencies.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.92%. Comparing base (dead429) to head (37ddf4c).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #298   +/-   ##
=======================================
  Coverage   41.92%   41.92%           
=======================================
  Files          14       14           
  Lines        2092     2092           
  Branches       42       42           
=======================================
  Hits          877      877           
  Misses       1197     1197           
  Partials       18       18           

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