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.

.gitignore

@@ -468,3 +468,6 @@ lerna-debug.log*
 # Windows thumbnail cache
 Thumbs.db
 ehthumbs.db
+
+# Self-installed .NET SDK (scripts/ci/lib.sh ensure_dotnet, used when a CI runner lacks the SDK)
+/.dotnet/