ci: node16-safe actions + GitHub-only gates for Gitea mirror (#25)

The Gitea act_runner runs action JS in node:16-bullseye; setup-go@v5 crashes
with 'ReferenceError: ReadableStream is not defined'. Gitea discovers
.github/workflows too, so:

- setup-go@v5 -> @v4 (node16-safe, fine on GitHub)
- gate GoReleaser (release.yml) + release-please jobs to
  github.server_url == https://github.com so the GitHub CD never runs on the
  Gitea mirror (GoReleaser/release-please are GitHub-release-centric)

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Set up Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@v4
         with:
           go-version: ${{ matrix.go }}
           cache: true