Fix mount loop test for bindings

test -f will fail if we wanted to bind to a directory,
so use test -r instead.

See FS#15356

Signed-off-by: Aaron Griffin <aaronmgriffin@gmail.com>

archiso/hooks/archiso

@@ -94,7 +94,7 @@ run_hook ()
 
         [ "$imgarch" != "$arch" ] && continue
 
-        [ ! -f "/bootmnt/${img}" ] && continue
+        [ ! -r "/bootmnt/${img}" ] && continue
 
         if [ "${type}" = "bind" ]; then
             _mnt_bind "/bootmnt/${img}" ${mountpoint}