(ci) Build on Ubuntu 20.04 instead of 22.04

There is an annoying problem here:

- The site depends on `pygments.rb`
- Pygments.rb in turn, communicates with a Python process using a pipe
  (reading and writing to stdin/stdout from Python running as a child
  process)
- The version of Pygments.rb we use (?) do not work with Python 3. Only
  Python 2 works.

Ubuntu 22.04 uses Python 3 by default (and this is perhaps the case with
Ubuntu 20.04 also), which causes errors like the one seen in
https://github.com/perlun/perlun.eu.org/actions/runs/5913115348/job/16037183000.
Copying it here into the git commit log, since Github Actions logs are
unfortunately not retained forever.

bundle exec jekyll build
       Deprecation: The 'gems' configuration option has been renamed to 'plugins'. Please update your config file accordingly.
Configuration file: /home/runner/work/perlun.eu.org/perlun.eu.org/_config.yml
            Source: /home/runner/work/perlun.eu.org/perlun.eu.org
       Destination: /home/runner/work/perlun.eu.org/perlun.eu.org/_site
 Incremental build: disabled. Enable with --incremental
      Generating...
Building site for default language: "xx" to: /home/runner/work/perlun.eu.org/perlun.eu.org/_site
Building site for language: "sv" to: /home/runner/work/perlun.eu.org/perlun.eu.org/_site/sv
Loading translation from file /home/runner/work/perlun.eu.org/perlun.eu.org/_i18n/sv.yml
Building site for language: "en" to: /home/runner/work/perlun.eu.org/perlun.eu.org/_site/en
  Conversion error: Jekyll::Converters::Markdown encountered an error while converting '_i18n/en/_posts/2017-03-17-new-features-in-c-sharp-6-0-and-7-0.md':
                    No header received back.
jekyll 3.9.2 | Error:  No header received back.

Author: perlun

.github/workflows/build.yml

@@ -4,7 +4,7 @@ on: [push]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2