Make cache use zstd compression

[bertptrs]

Update the cache command to create Zstd-compressed archives rather than gzip-compressed. In testing, this reduces archive size by 20%, with proportional gains in download times, as well as a 50% reduction in decompression time.

This change is backwards-compatible and can handle existing, gzip-compressed archives without changes. To do this, we attempt to see if the file is valid Zstd. For the shell-out archiver, I opted to let tar's built-in autodetection handle the differentiation. This works on all tested systems.

Tested manually on Semaphore Linux- and MacOS cloud runners. Timing improvements measured on our own hardware as Semaphore runners appear to be I/O bound.

Fixes #523. In the future, you might want to delete the old gzip implementation as it's messy.

[bertptrs]

Note that the Go zstd implementation is not particularly fast, so the shell-out archiver becomes faster than the native-parallel archiver with this change. What to do with this is an exercise to the reader. It still beats gzip regardless.