cleanup build caching

[Gankra]

• because we pass RUSTFLAGS(?) we now always completely rebuild crash-client and minidumper-test

  • easy win, stop trying to get symbols of minidumper-test, they're useless
  • hard win, setup profiles? will that fix caching? can you use custom profiles with --install?

• because path deps are a hack on top of cargo build (to get incremental), switching between path-and-not-path causes cargo install to freak out about binaries existing it didn't expect

  • when doing a path build setup the metadata files cargo install expects?

• because we have a global binary/build dir, changing pipelines requires clobbering and completely rebuilding any changed tools

  • we could move build-dirs into the /runs/$RUN dir but...
    • this currently gets nuked (we could more selectively delete everything else)
    • this would have the reverse problem of "things that don't change" not getting shared and rebuilding when doing two separate runs
    • i think The Dream is that we have a global build dir that is keyed in some way that different things can be different and same things can be same