Enforce C.UTF-8 locale for all commands we run

Enforce C.UTF-8 locale for all commands we run

So this is a shitty situation either way. We generally have no idea
which locales will be available in the tools tree. Most likely, it'll
just be the C.UTF-8 one. In that case, using the locale environment
variables from the host is pointless and leads to annoying errors from
lots of tools.

We could make sure all locales are installed but glibc-all-langpacks is
> 200M installed size on Fedora which feels like too much. So let's
instead always use C.UTF-8, it's not like mkosi itself is locale aware
anyway.