Lock the cache to a specific rustc + dependencies

Apparently, GitHub doesn't write back to the cache, leading to massive 
build times a few moments after its initialization (when a change 
happens invalidating it). While this forces a new cache whenever 
dependencies change, it'll restore from an older set of dependencies in 
that case, still minimizing build times.

.github/actions/cached-rust/action.yml

@@ -22,7 +22,8 @@ runs:
           ~/.cargo/registry
           ~/.cargo/git
           ./target
-        key: ${{ runner.os }}-${{ runner.arch }}-rust-${{ inputs.toolchain }}
+        key: ${{ runner.os }}-${{ runner.arch }}-rust-${{ steps.install-rust.outputs.rustc_hash }}-${{ hashFiles("**/Cargo.lock") }}
+        restore-keys: ${{ runner.os }}-${{ runner.arch }}-rust-${{ steps.install-rust.outputs.rustc_hash }}-
 
     - name: Install Rust
       uses: actions-rs/toolchain@v1