Update build.yml

Author: josteink

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-18.04, ubuntu-latest]
         emacs: [26.1, 26.2, 26.3, 27.1, 27.2, snapshot]
 
     steps: