Disable git clean on self-hosted runners to preserve build cache

actions/checkout@v4 defaults to clean: true, which runs git clean -ffdx.
This follows the build/ symlink into the shared cache directory and
deletes all cached artifacts (staging, install, venv), defeating the
purpose of the persistent cache and causing SIGILL errors from partially
destroyed build artifacts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/test.yml

@@ -210,6 +210,8 @@ jobs:
     steps:
       - name: Clone
         uses: actions/checkout@v4
+        with:
+          clean: false
 
       - name: Build
         if:   matrix.cluster != 'phoenix'