Enable caching to save time by recompilation

[erikscheurer]

Currently, some form of caching is implemented, however, it does not work at the moment. The caching step throws an error:

Warning: Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.
Warning: Cache save failed.

even though $HOME/precice exists.

In #1 a few tries also lead showed that it did not work as expected:
When hard coding /home/runner/precice/ the cache worked as expected, however, the cache failed to be downloaded again, even though the key was unique (Linux-precice-5e07bad3ac6fda7e761aacf80461942b2f83ceca)

What also should be changed about the caching is the key, which should include the build directory to restore to the correct location.