Update build.yml

Author: ethteck

.github/workflows/build.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-20.04, macos-latest, windows-latest]
         ido: [5.3, 7.1]
 
     name: Recompiling ido ${{ matrix.ido }} for ${{ matrix.os }}