Our mkosi won't work with Fedora 33

79eee02a sets the mkosi version at a fixed version which won't work with Fedora 33:

 File "mkosi/mkosi", line 1544, in install_fedora
    gpg_key = "https://getfedora.org/static/keys/{}.txt".format(FEDORA_KEYS_MAP[args.releasever])
KeyError: '33'

Upstream patch to mkosi fixes the key error but that came after the sha we're hardcoding to.

We should figure out if mkosi can be updated yet or not.