systemd/kres-cache-gc: use correct cache directory

In 5.0.0, the cache directory has been moved and the previous path is incorrect.

Fixes #543 (closed)

This issue wasn't caught even though we have automated test for kres-cache-gc.service. The service is active/running, but it basically doesn't do anything and just reports errors to the log.

Changing the test to check journal for error messages doesn't seem reliable.

The current design of kres-cache-gc seems to be suitable for standalone daemon, which we want to keep running if at all possible. Perhaps we could modify kres-cache-gc to have a different mode for systemd, when it would crash on all errors, and then systemd could be used to restart it?

In that case we would have to be careful about initial conditions, say the GC not starting (and failing) before cache file is created. That might be possible thanks to sd_notify().

I can't think of an easy way of automatic testing that GC really works (say, keeps the cache from overflowing).

merged

mentioned in commit 5a9c77ed

Let's not block merging the fix itself.