CI cache is large

[clehner]

Cache in this run failed due to out of disk space: https://github.com/spruceid/ssi/runs/2240675344?check_suite_focus=true#step:3:27

Cache Size: ~2888 MB (3028672707 B)

Maybe DIDKit's larger cache got mixed in here? #140 was supposed to prevent that. Or maybe the cache just got big from a lot of old build artifacts.

We could increment a version id in the cache key to trigger refreshing the cache: https://stackoverflow.com/questions/63521430/clear-cache-in-github-actions/64819132#64819132