github: switch whitespace-errors job to ubuntu-latest

While ubuntu-latest in ci is not necessarily the latest version of the
OS available in ci, whitespace-errors job should be agnostic to the
version being used, so use the default one.

Author: ldv-alt

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   whitespace-errors:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - name: check