Research using relative path everywhere for `tar` in `save_cache`

[spencertransier]

When adding the install_swiftpm_dependencies action, I added a tar option in the save_cache action that uses a relative path. See this comment thread for details: #43 (comment)

Existing tar usage: tar -czf "$CACHE_KEY" "$CACHE_FILE"
Relative path: tar -czf "$CACHE_KEY" -C "$CACHE_FILE" .

Let's research whether the relative path example could be used by all the instances of save_cache in the A8C code base.

[AliSoftware]

Encountered this issue when I worked on fixing publish_pod to add support for --synchronous when publishing co-dependent podspecs from a single repo. See #85

TL;DR: pod trunk push --synchronous relies on using CocoaPods' master spec repo instead of the CDN (to get rid of CDN propagation latencies), and the master repo being very large I tried to use cache_cocoapods_specs_repos action for such case. But it ended up failing to restore any file from the cache tarball seemingly to that exact relative-path vs absolute-path issue.

It's very possible that using -C during cache_cocoapods_specs_repos might help solve this issue. I don't have the bandwidth to try rn though, but definitively worth checking at some point.