Revert the GitHub workflow guard to server_url

Misdiagnosed the earlier report: Gitea was correctly running its own
.gitea/workflows copy, not this .github one, so the server_url guard was fine.
The switch to a GITHUB_RELEASE repo variable then made the workflow skip on
GitHub itself, since that variable is not set there.

Put both job guards back to `github.server_url == 'https://github.com'`, which
runs on GitHub with no manual setup and skips on Gitea (its .gitea copy is the
one that runs). No repository variable is needed.

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

.github/workflows/release.yml

@@ -5,15 +5,10 @@ name: Release
 # macOS and Windows); only the publish half differs: GitHub needs the v4
 # 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`. (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.
+# Every job is gated on `github.server_url == 'https://github.com'`, so this file
+# runs only on GitHub. Gitea runs its own copy in .gitea/workflows; should a
+# Gitea instance also read .github/workflows, the guard skips these jobs there
+# so they never fail on the @v4 artifact backend or `gh`.
 on:
   push:
     tags: ["v*"]
@@ -28,7 +23,7 @@ env:
 jobs:
   build:
     name: ${{ matrix.asset }}
-    if: ${{ vars.GITHUB_RELEASE == 'true' }}
+    if: ${{ github.server_url == 'https://github.com' }}
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
@@ -149,7 +144,7 @@ jobs:
   publish:
     name: Publish release
     needs: build
-    if: ${{ vars.GITHUB_RELEASE == 'true' }}
+    if: ${{ github.server_url == 'https://github.com' }}
     runs-on: ubuntu-latest
     permissions:
       contents: write