pmb/chroot/binfmt: don't register arch if OS does it automatically (MR 2088)

It seems like the gitlab runners will now automatically register archs
when binfmt is mounted (I'm not completely sure, it's really
hard/annoying to confirm using gitlab CI jobs and no direct access to
the runner)

In any case, checking if it's already registered fixes a problem where
CI fails with "File exists" when it tries to register the arch.

pmb/chroot/binfmt.py

@@ -22,6 +22,12 @@ def register(args, arch):
         return
     pmb.helpers.other.check_binfmt_misc(args)
     pmb.chroot.apk.install(args, ["qemu-" + arch_qemu])
+
+    # Don't continue if the actions from check_binfmt_misc caused the OS to
+    # automatically register the target arch
+    if is_registered(arch_qemu):
+        return
+
     info = pmb.parse.binfmt_info(args, arch_qemu)
 
     # Build registration string