CI: Run multiarch tests on ubuntu-latest

The cross-architecture tests fail in glib2:

    Exec failed with: Failed to close file descriptor for child process (Operation not permitted)

That's the 2-year-old GitHub bug
<actions/runner-images#3812>. Some other
projects which run ubuntu-latest reported that the bug is already
fixed. We used ubuntu-20.04. ubuntu-latest is ubuntu-22.04. Try that.

.github/workflows/multiarch.yaml

@@ -3,11 +3,11 @@ on: [push, pull_request]
 
 jobs:
   multiarch:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     name: ${{ matrix.distro }} on ${{ matrix.arch }}
     continue-on-error: true
 
-    # Run steps on a matrix of 3 arch/distro combinations
+    # Run steps on a matrix of 2 arch/distro combinations
     strategy:
       matrix:
         include: