Use Ubuntu 18.04 for the workflow

[why]
There are currently problems at Github to use ubuntu-latest workflows.

[how]
Use 18.04 as this is enough for us at the moment.
This needs to be changed back sometimes...

[note]
There seems to be no font-patcher PPA for 20.04.

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

.github/workflows/generate-fonts.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       NERDFONTVERS: v2.1.0
     steps: