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

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