Try CI on ubuntu instead

Author: simonw

.github/workflows/test.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: macos-latest
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]