Update workflow to use ubuntu-latest

Author: nside

.github/workflows/publish.yaml

@@ -7,7 +7,7 @@ on:
 jobs:
   build-n-publish:
     name: Build and publish Python 🐍 distributions 📦 to PyPI
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v2