GHA: Reduce cache size

[dweindl]

We have limit cache size. Only cache what's really relevant.

[codecov-commenter]

Codecov Report

Attention: 384 lines in your changes are missing coverage. Please review.

Comparison is base (160c2a8) 88.16% compared to head (b0502cc) 83.86%. Report is 481 commits behind head on develop.

Files	Patch %	Lines
pypesto/ensemble/ensemble.py	69.77%	68 Missing :warning:
...ypesto/hierarchical/spline_approximation/solver.py	82.62%	53 Missing :warning:
pypesto/ensemble/util.py	65.13%	38 Missing :warning:
pypesto/hierarchical/optimal_scaling/solver.py	93.53%	26 Missing :warning:
pypesto/history/base.py	88.57%	24 Missing :warning:
pypesto/hierarchical/petab.py	84.76%	23 Missing :warning:
pypesto/engine/mpi_pool.py	0.00%	22 Missing :warning:
pypesto/hierarchical/inner_calculator_collector.py	88.51%	17 Missing :warning:
pypesto/hierarchical/problem.py	89.36%	15 Missing :warning:
...pesto/hierarchical/spline_approximation/problem.py	89.65%	15 Missing :warning:
... and 15 more

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1261 +/- ## =========================================== - Coverage 88.16% 83.86% -4.31% =========================================== Files 79 148 +69 Lines 5257 12067 +6810 =========================================== + Hits 4635 10120 +5485 - Misses 622 1947 +1325 ```

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[dweindl]

What does ~/.cache/pip change? The tests cache somewhere else? Or cache less? (for my info)

It's just the directory to be cached. Previously is was all under ~/.cache, now it's only ~/.cache/pip.