mkosi: switch to github mirror of openSUSE sources from build.opensuse.org (#35789)

The commit history of
[src.opensuse.org](https://src.opensuse.org/pool/systemd/src/branch/devel)
has been reset again to 2015 (both branches, factory and devel)... this
is unusable for now. I'd suggest switching to the github mirror of
code.opensuse.org, maintained since 2019 (at least github is not down
that often and the commit history does not change).