Hash the Cargo.toml instead of the Cargo.lock for cache

We don't commit a .lock file because Physis is a library, so the .toml
is the next best thing.

.github/workflows/main.yml

@@ -31,7 +31,7 @@ jobs:
           ~/.cargo/registry/cache/
           ~/.cargo/git/db/
           target/
-        key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+        key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.toml') }}
     - name: Install dependencies
       run: |
         sudo apt-get update