Revert "Update ubuntu runner to 'ubuntu-24.04'"

This reverts commit 2959505.
We prefer to stick with older ubuntu releases for building because of
containers#256
The issue with qemu with ubuntu 20.04 was fixed with
containers@fe7285a8a6a

Ubuntu 20.04 will be EOL'ed in April and the runner images will also go
away so we'll need to revisit this:
actions/runner-images#11101

Some options are to switch to Ubuntu 22.04 for our builds, or to look
into having pure go builds or statically linked builds, ...

Signed-off-by: Christophe Fergeau <[email protected]>

.github/workflows/go.yml

@@ -9,7 +9,7 @@ on:
 jobs:
 
   build:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-20.04 # explicitly use 20.04, see commit 428c40018a
     timeout-minutes: 30
     strategy:
       fail-fast: false