Downgrade Ubuntu version for Arm testing

Our Arm jobs have been failing for a while. At first that was sporadic
(in retrospect likely because we got runners sporting different images
due to staged rollout of a new Ubuntu version) and now its permanent.
The cause of the failure seems to be some change in format of their
sources.list, causing us to no longer be able to no longer fetch
necessary package lists. I haven't found a guide in their documentation
how to properly update, so roll back to a known good distro version
until somebody has the nerves to deal with this %#&*($&#*($.

Signed-off-by: Daniel Müller <[email protected]>

.github/workflows/test.yml

@@ -153,7 +153,7 @@ jobs:
 
   build-aarch64:
     name: Build for aarch64
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     # Make this job optional until Ubuntu gets their act together and
     # provides usable infrastructure.
     continue-on-error: true
@@ -184,7 +184,7 @@ jobs:
 
   build-armhf:
     name: Build for aarch32
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     # Make this job optional until Ubuntu gets their act together and
     # provides usable infrastructure.
     continue-on-error: true