Replace setup-go with a custom action

To maximize the caching efficiency, all build jobs are executed as
forgejo user.
Hash is explicitly specified to avoid breaking changes.

.forgejo/workflows/composite/setup-env/action.yaml

@@ -1,11 +1,11 @@
 runs:
   using: "composite"
   steps:
-    - uses: https://code.forgejo.org/actions/setup-go@v4
-      with:
-        go-version-file: "go.mod"
     - name: setup user and permissions
       run: |
         git config --add safe.directory '*'
         adduser --quiet --comment forgejo --disabled-password forgejo
         chown -R forgejo:forgejo .
+    - uses: https://codeberg.org/fnetx/setup-cache-go@b2214eaf6fb44c7e8512c0f462a2c3ec31f86a73
+      with:
+        username: forgejo

.forgejo/workflows/testing.yml

@@ -214,5 +214,5 @@ jobs:
     steps:
       - uses: https://code.forgejo.org/actions/checkout@v3
       - uses: ./.forgejo/workflows/composite/setup-env
-      - run: make deps-backend deps-tools
-      - run: make security-check
+      - run: su forgejo -c 'make deps-backend deps-tools'
+      - run: su forgejo -c 'make security-check'