Add BuildSourcesEphemeral=

Add BuildSourcesEphemeral=

Unfortunately there are use cases where it's useful to persist
changes to the source directory. A prime example is projects with
Makefile that do not provide a properly functioning `make install`.
The only way to use those with mkosi build scripts is to do an
in-tree build in the build script and then mounting the source
directory into the VM or container when booting it. For this to work
we need changes to the source directory to be persisted. To support
this use case, let's gate the ephemeral source directories behind an
option that's disabled by default (for backwards compatibitity reasons).