mkosi: default to Fedora 38

mkosi: default to Fedora 38

It'll be out this week. We can't update the man pages before it is realeased,
but we can use it for mkosi builds and do some very late testing.

Also, use filepath specification for /bin/pkg-config. We need it for meson, and
meson calls it directly by this path. pkgconfig is a virtual Provides on
pkgconf-pkg-config, and the indirection here just obfuscates things with no
benefit.

Add it explicitly for centos too. (I think it is pulled in by packages which
contain pkg-config modules anyway, but it's better to be explicit).