mkosi: if we build a read-only image, use nspawn's --overlay= switch to make /root/src writable
mkosi: if we build a read-only image, use nspawn's --overlay= switch to make /root/src writable

This way, we can build on the source tree even if the disk image is
read-only. This primarily useful for squashfs where the file system
can#t be made writable even if we wanted to. We turn this on for all
kind of read-only images however, to keep the codepaths similar, and to
isolate the build artifacts from the disk image we build from.

This makes use of the new nspawn --overlay=+/root/src::/root/src syntax
added by #4694.

UbiCastTeam/mkosi,UbiCastTeam/mkosi,systemd/mkosi,systemd/mkosi