Change CI workflow to use ubuntu-20.04 for consistency in test environment

Author: BraisVQ

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 jobs:
   tailor:
     name: Run tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       -
         name: Checkout repository