mkosi: Re-enable arch but disable keyring checking

No need to disable arch completely, let's just disable keyring checking
to get CI working again for now.

.github/workflows/mkosi.yml

@@ -54,9 +54,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-# Temporarily disabled due to keyring packaging issues
-#          - distro: arch
-#            release: rolling
+          - distro: arch
+            release: rolling
           - distro: debian
             release: testing
           - distro: ubuntu
@@ -104,6 +103,15 @@ jobs:
         Ephemeral=yes
         EOF
 
+        # TODO: Remove once archlinux-keyring is updated in ppa:michel-slm/kernel-utils.
+        tee mkosi.conf.d/11-arch-keyring.conf <<- EOF
+        [Match]
+        Distribution=arch
+
+        [Distribution]
+        RepositoryKeyCheck=no
+        EOF
+
         # For erofs, we have to install linux-modules-extra-azure, but that doesn't match the running kernel
         # version, so we can't load the erofs module. squashfs is a builtin module so we use that instead.