build: migrate build runner from ubuntu-20.04 to ubuntu nightly

Support for Ubuntu 20.04 on build runners as been discontinued by GitHub, hence pipelines are getting cancelled.

see: [https://github.com/actions/runner-images/issues/11101](https://github.com/actions/runner-images/issues/11101), for full
info

Author: 50-Course

.github/workflows/test.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
     name: build (Python ${{ matrix.python-version }}, Django ${{ matrix.django-version }})
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix: