ci: use upload-artifact v3 for gitea

Assisted-by: Delta:zai-glm-5.3

.gitea/workflows/build.yml

@@ -16,8 +16,13 @@ jobs:
       - name: Build site
         run: ./scripts/calepin.sh compile site public
 
+      # The official upload-artifact@v4 refuses to run on Gitea (it detects a
+      # non-GitHub server and aborts with GHESNotSupportedError). v3 uses the
+      # artifact API that Gitea implements. On Gitea >= 1.22 with a current
+      # act_runner, christopherhx/gitea-upload-artifact@v4 is a drop-in
+      # replacement if you want the v4 API.
       - name: Upload site artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: public
           path: public/