simplify CI cache

[ranocha]

This uses a new GitHub action https://github.com/marketplace/actions/cache-julia-artifacts-packages-and-registry made specifically for this purpose, see also https://discourse.julialang.org/t/recommendation-cache-julia-artifacts-in-ci-services/35484/8

[codecov]

Codecov Report

Merging #123 (f3f45b3) into main (7498834) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #123   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files          30       30           
  Lines        4742     4742           
=======================================
  Hits         4360     4360           
  Misses        382      382           

Flag	Coverage Δ
unittests	91.94% <ø> (ø)

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[coveralls]

Coverage remained the same at 91.944% when pulling f3f45b3 on ranocha-patch-1 into 7498834 on main.