fix to ubuntu work flow

Author: melsman

.github/workflows/main.yml

@@ -17,7 +17,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-20.04, macos-latest]
         mlcomp: [mlkit, mlton]
 
     runs-on: ${{ matrix.os }}