Be explicit about GH runner versions.

Author: toinehartman

.github/workflows/build.yaml

@@ -49,7 +49,7 @@ jobs:
   ui-test:
     strategy:
       matrix:
-        os: [ubicloud-standard-4, ubicloud-standard-4-ubuntu-2204, ubuntu-latest]
+        os: [ubicloud-standard-4, ubicloud-standard-4-ubuntu-2204, ubuntu-22.04, ubuntu-24.04]
         delay_factor: [8, 16, 32, 64]
       fail-fast: false
     env: