Improve node_modules caching

This attempts to have the cache key derived from package-lock.json's
hash.

action.yml

@@ -10,13 +10,19 @@ inputs:
 runs:
   using: composite
   steps:
+    # We need to copy codemention's package-lock.json into the workspace
+    # because hashFiles only works on files inside the workspace.
+    - run: cp ${{ github.action_path }}/package-lock.json codemention-package-lock.json
+      shell: bash
     - uses: actions/cache@v3
+      id: cache-node-modules
       with:
         path: ${{ github.action_path }}/node_modules
-        key: codemention-npm
+        key: codemention-npm-${{ hashFiles('codemention-package-lock.json') }}
     - run: npm install
       working-directory: ${{ github.action_path }}
       shell: bash
+      if: steps.cache-node-modules.outputs.cache-hit != 'true'
     - run: npm run build
       working-directory: ${{ github.action_path }}
       shell: bash