This is redundant with the "run_tests" workflow, which is now just as
fast since adding sccache.

* Incremental builds don't help here
* sccache won't cache incrementally compiled crates
* this is also set in the "run_tests" workflow, where the corresponding
  configuration shares cache with this job. Using different
  configurations may cause the caches to be incompatible (i.e. cause
  cache misses)

I don't think SCCACHE_RECACHE would have worked as intended in this
scenario. It tells the sccache server to reset its cache, but the CI
wouldn't have written back the cache anyway since the cache key didn't
change.

Instead we can increment SCCACHE_CACHE_VERSION to force new caches to be
saved.