crux-mir-build.yml: Cache cargo dependencies

[RyanGlScott]

Fixes #1160.

[sauclovian-g]

Seems plausible

[RyanGlScott]

Hm, this appears not to be caching things like I would expect them to. I re-ran a job to test the caching, and it did record a cache hit:

Cache hit for: 1-ubuntu-22.04-cargo-cache-Linux-7fbb90a9-31b6338a
Received 25165824 of 507566794 (5.0%), 24.0 MBs/sec
Received 134217728 of 507566794 (26.4%), 63.8 MBs/sec
Received 268435456 of 507566794 (52.9%), 85.0 MBs/sec
Received 402653184 of 507566794 (79.3%), 93.2 MBs/sec
Received 507566794 of 507566794 (100.0%), 96.7 MBs/sec
Cache Size: ~484 MB (507566794 B)
/usr/bin/tar -xf /home/runner/work/_temp/a4e98288-cf8a-4e74-a7a8-c9e5b66d0180/cache.tzst -P -C /home/runner/work/crucible/crucible --use-compress-program unzstd
Cache restored successfully
Restored from cache key "1-ubuntu-22.04-cargo-cache-Linux-7fbb90a9-31b6338a" full match: true.

On the other hand, when it installs mir-json, it recompiles all of its dependencies!

   Compiling libc v0.2.126
   Compiling pkg-config v0.3.17
   Compiling autocfg v0.1.7
   Compiling cfg-if v0.1.10
   Compiling proc-macro2 v1.0.39
   Compiling unicode-ident v1.0.0
   Compiling syn v1.0.96
   Compiling quote v1.0.2
   Compiling jobserver v0.1.24
   Compiling cc v1.0.83
   ...

I wonder if I have misconfigured something with the rust-cache action?

[RyanGlScott]

I fixed this by adding a workspaces: "dependencies/mir-json" field to the action's stanza in the YAML file, which allows rust-cache to figure out what to cache.