Archive the cached dir relative, not by its absolute path

Each run gets a fresh per-run workspace (~/.cache/act/<hash>/hostexecutor),
so embedding the absolute path meant the next run restored into a stale
workspace and the cache never actually sped up a build. Save with
-C <parent> <basename> so entries are <basename>/... and restores can
extract into the run's own path input location.

README.md

@@ -23,6 +23,9 @@ A Gitea Action that caches build files as `tar.xz` archives on the runner filesy
 2. Stores archives at `~/.cache/.cache-store/<key>.tar.xz`
 3. Restores by extracting the archive to `/`, saves by creating one
 4. Saves atomically (write to `<key>.tar.xz.tmp`, then rename)
+5. Archives store the cached directory **relative** (`<basename>/...`), not its
+   absolute path — a later run restores into its own `path` location, so the
+   cache works across runs even though each run gets a fresh workspace
 
 ## Cache Retention