Bump Ubuntu to 20.04 in test enviroment, for Cortex-M33 (L5) support.

.github/workflows/cmake.yml

@@ -8,7 +8,7 @@ env:
 
 jobs:
   test-ubuntu:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         family: [F0, F1, F2, F3, F4, F7, G0, G4, H7, L0, L1, L4, L5]