actions: revert to ubuntu-20 for e2e tests (#1140)

* The Ubuntu 22 image does not have Firefox pre-installed, yet...
* Revert to ubuntu-20 until it is reinstated.

Author: oliver-sanders

.github/workflows/main.yml

@@ -56,7 +56,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             browser: firefox
           - os: ubuntu-latest
             browser: chrome