Add support for building on OBS

These minimal changes allow building an Arch package using the recipe and the upstream sources from main on SUSE's build service:

https://build.opensuse.org/package/show/home:bluca:branches:home:bluca:systemd/mkosi

Unfortunately there's no nice way to detect we are building on this system, but there are some quite unique heuristics that can be used in the form of specific config files and directories, which is good enough for this purpose.

Same as systemd!27 (merged)