chore(hermit): share one CARGO_HOME across worktrees (#8250)

🤖 Hermit defaults `CARGO_HOME` to each checkout's `.hermit/rust`, so
every worktree sees dependency sources at a different absolute path.
sccache keys Rust compiles on those paths, which means the same
dependency built in two worktrees almost never hits the cache. This pins
`CARGO_HOME` to `${HOME}/.cargo` in `bin/hermit.hcl`, the same setting
buzz-app already uses.

CI also lines up better: the workflows that activate Hermit and cache
`~/.cargo/registry` / `~/.cargo/git` were caching a directory cargo
wasn't using (cargo wrote to `$GITHUB_WORKSPACE/.hermit/rust`). With
this change those cache paths are where cargo actually puts the registry
and git checkouts. The `MESH_ROOT` lookups in `_ci-desktop-macos.yml`,
`release.yml`, and `signed-macos-canary.yml` already read
`${CARGO_HOME:-$HOME/.cargo}`, so they follow either way.

Signed-off-by: Will Pfleger <pfleger.will@gmail.com>
Co-authored-by: Duncan <dcfd242e557282d7a1e2cf2e6877522682f1e5c6156dc92ca7d90eaedd3b0f95@buzz.block.builderlab.xyz>

bin/hermit.hcl

@@ -1 +1,5 @@
 manage-git = true
+
+env = {
+  "CARGO_HOME": "${HOME}/.cargo",
+}