fix: set GLOBAL mirror before mcpp test (gtest download on GH runners)

When cache misses (e.g., .xlings.json changed), gtest needs fresh
download. Default CN mirror is unreachable from GitHub runners.
Set GLOBAL mirror before test step, same as E2E already does.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -95,6 +95,8 @@ jobs:
 
       - name: Unit + integration tests via `mcpp test`
         run: |
+          # Ensure GLOBAL mirror for package downloads (runner is outside CN)
+          "$MCPP" self config --mirror GLOBAL
           "$MCPP" test
 
       - name: E2E suite