ci: Use restore-key to continually update ccache storage

[paulgessinger]

We observed a brief period of speedup in the CI after introducing ccache, which degraded again after a few weeks:

This is due to the fact that GitHub Actions caches do not get updated at the end of their run. This PR introduces a workaround. The CI will

1. Look for a key like cache_key_$OS_$JOB_$COMMIT_SHA and use that if found. If not it will
2. Look for the newest key starting with cache_key_$OS_$JOB_, and use that if found. Otherwise it will perform a build from scratch
3. At the end of the job, if a cache using the restore key (i.e. 2. above) was used, it will create a new cache with the key from 1 (cache_key_$OS_$JOB_$COMMIT_SHA). The next CI run will use that cache as a starting point.

Clearly, this means that the cache size will continually increase. However, ccache has a maximum size setting, which I set to 2GB in this PR. We'll have to see if that results in a reasonably sized, reasonably up-to-date cache and corresponding fast build times.

[codecov]

Codecov Report

Merging #1263 (3324a56) into main (6eb3937) will increase coverage by 0.40%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1263      +/-   ##
==========================================
+ Coverage   47.31%   47.71%   +0.40%     
==========================================
  Files         375      375              
  Lines       19758    19718      -40     
  Branches     9345     9264      -81     
==========================================
+ Hits         9348     9409      +61     
+ Misses       4021     3920     -101     
  Partials     6389     6389              

Impacted Files	Coverage Δ
Core/include/Acts/Seeding/SeedfinderConfig.hpp	0.00% <0.00%> (ø)
Core/include/Acts/Propagator/EigenStepper.ipp	50.73% <0.00%> (+0.37%)	⬆️
Core/src/TrackFitting/GainMatrixSmoother.cpp	10.86% <0.00%> (+0.45%)	⬆️
Core/include/Acts/Propagator/ConstrainedStep.hpp	74.50% <0.00%> (+7.20%)	⬆️
Core/include/Acts/Propagator/AtlasStepper.hpp	71.01% <0.00%> (+8.95%)	⬆️

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[andiwand]

great 👍