Use a separate action to calculate hashes

`hashFiles` is broken in `act`, see https://gitea.com/gitea/act_runner/issues/40

.gitea/workflows/ci.yaml

@@ -10,11 +10,16 @@ jobs:
         things: [1, 2, 3, 4, 5]
     steps:
       - uses: "https://github.com/actions/checkout@v3"
+      - name: Hash the files
+        uses: theowenyoung/folder-hash@v3
+        id: hash-stuff
+        with:
+          path: .gitea/workflows/ci.yaml
       - name: Cache stuff
         uses: "https://github.com/actions/cache@v3"
         with:
           path: .
-          key: stuff-${{ hashFiles('.gitea/workflows/ci.yaml') }}
+          key: stuff-${{ steps.hash-stuff.outputs.hash }}
       - run: sleep 5
       - name: Hello world
         run: docker run hello-world