fix: use github.com source for setup-java and set Forgejo server URL

- Change actions/setup-java@v4 to https://github.com/actions/setup-java@v4
  (not mirrored on code.forgejo.org)
- Add FORGEJO_SERVER_URL env var set to https://srvr.nu/git
  (runner checkout was missing /git/ subpath prefix)

.forgejo/workflows/ci.yml

@@ -6,6 +6,9 @@ on:
   pull_request:
     branches: [master, develop]
 
+env:
+  FORGEJO_SERVER_URL: https://srvr.nu/git
+
 jobs:
   lint-and-test:
     name: Lint, type check, unit tests, coverage
@@ -19,7 +22,7 @@ jobs:
           cache: npm
           cache-dependency-path: frontend/package-lock.json
 
-      - uses: actions/setup-java@v4
+      - uses: https://github.com/actions/setup-java@v4
         with:
           distribution: temurin
           java-version: 21