chore: Test bash-5.0 using nix instead of ubuntu-20

ubuntu-20.04 isn't available anymore, so the trick of using it to test
an older version of bash doesn't work.

This change switches to downloading an older version of bash using nix
on the current version of ubuntu.

Author: szermatt

.github/workflows/test.yml

@@ -37,8 +37,8 @@ jobs:
     - name: Test the project
       run: 'eldev -p -dtT test'
 
-  test-ubuntu-20:
-    runs-on: ubuntu-20.04 # bash 5.0
+  test-bash-5_0:
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         emacs_version:
@@ -50,6 +50,11 @@ jobs:
       with:
         version: ${{matrix.emacs_version}}
 
+    - name: Install bash 5.0
+      run: |
+        nix profile install nixpkgs/3b05df1d13c1b315cecc610a2f3180f6669442f0#bash
+        which bash
+
     - name: Install Eldev
       uses: emacs-eldev/setup-eldev@v1