mkosi: temporarily disable Arch

The mkosi Arch CI doesn't work as the keyring package is out
of date and cannot be built due to various build toolchain
issues. Disable the job as it always fails and confuses
submitters.

.github/workflows/mkosi.yml

@@ -54,8 +54,9 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - distro: arch
-            release: rolling
+# Temporarily disabled due to keyring packaging issues
+#          - distro: arch
+#            release: rolling
           - distro: debian
             release: testing
           - distro: ubuntu