Auto-install .NET SDK in CI scripts when runner lacks it

Gitea runner build failed with "dotnet: command not found". Added
ensure_dotnet() to lib.sh, using the vendored dotnet-install.sh to
bootstrap the SDK into .dotnet/ when not already on PATH, called from
build.sh/test.sh/prepush.sh. Also trigger CI on build-runner-fix to
verify the fix.

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ name: CI
 
 on:
   push:
-    branches: [main]
+    branches: [main, build-runner-fix]
 
 jobs:
   build-and-push: