🩹 Encode runner in the cargo-install cache key

The cache key used for the `cargo-install` component action was a
generic string of `cargo-install-<target>`, which did not encode the
OS that the runner is running on. This appeared to be fine for a while,
but is now failing between macOS and Linux; with my suspicions being
that this is due to it allowing cross-os caching for Linux and macOS.
The docs for the cross-OS feature only calls out Windows as the
cross-target, so it's unsure if this is actually the reason.

At any rate, the fix is easy enough, and for correctness this is
what should have been done anyway.

.github/actions/cargo-install/action.yaml

@@ -18,7 +18,7 @@ runs:
       uses: actions/cache@v3
       with:
         path: .cargo/bin/${{ inputs.target }}
-        key: cargo-install-${{ inputs.target }}
+        key: cargo-install-${{ runner.os }}-${{ inputs.target }}
 
     - name: Install cargo binary
       if: steps.cargo-install.outputs.cache-hit != 'true'