Run CI and release builds one at a time

.gitea/workflows/ci.yaml

@@ -4,6 +4,11 @@ on:
   push:
     branches: ["**"]
 
+# One build at a time on the shared runner: a second run waits for the first.
+concurrency:
+  group: gites-build
+  cancel-in-progress: false
+
 jobs:
   check:
     runs-on: ubuntu-latest

.gitea/workflows/release.yaml

@@ -9,6 +9,11 @@ on:
 permissions:
   contents: write
 
+# One build at a time on the shared runner: a second run waits for the first.
+concurrency:
+  group: gites-build
+  cancel-in-progress: false
+
 jobs:
   release:
     runs-on: ubuntu-latest