Gate the GitHub release workflow on a repo variable, not server_url

The github.server_url guard did not keep this workflow from running on Gitea:
Gitea reports github.server_url as https://github.com, so the condition was
true there too and the jobs ran (and would fail on @v4/gh).

Switch both job guards to `vars.GITHUB_RELEASE == 'true'`, a repository variable
set only in the GitHub repo's settings. It is deterministic: absent on Gitea, so
the jobs skip there and the .gitea workflow is the one that runs; present on
GitHub once configured, so it runs there. The header documents how to set it
(Settings → Secrets and variables → Actions → Variables → GITHUB_RELEASE=true).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -6,9 +6,14 @@ name: Release
 # artifact backend and fills the release with `gh`.
 #
 # Gitea reads both .gitea/workflows and .github/workflows, so without a guard it
-# would also try to run this file and fail on @v4 and `gh`. Every job is gated
-# on running on github.com, so on Gitea they are simply skipped — the Gitea file
-# is the one that runs there.
+# would also try to run this file and fail on @v4 and `gh`. (Gitea reports
+# github.server_url as https://github.com, so that cannot tell the two apart.)
+# Instead every job is gated on a repository variable GITHUB_RELEASE=true, which
+# is set only in the GitHub repository's settings — so on Gitea, where it is
+# absent, these jobs are skipped and the .gitea file is the one that runs.
+#
+# To enable on GitHub: Settings → Secrets and variables → Actions → Variables →
+# New variable, name GITHUB_RELEASE, value true.
 on:
   push:
     tags: ["v*"]
@@ -23,7 +28,7 @@ env:
 jobs:
   build:
     name: ${{ matrix.asset }}
-    if: ${{ github.server_url == 'https://github.com' }}
+    if: ${{ vars.GITHUB_RELEASE == 'true' }}
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
@@ -144,7 +149,7 @@ jobs:
   publish:
     name: Publish release
     needs: build
-    if: ${{ github.server_url == 'https://github.com' }}
+    if: ${{ vars.GITHUB_RELEASE == 'true' }}
     runs-on: ubuntu-latest
     permissions:
       contents: write