Allow overriding ostree-build-yocto

If you create a $workdir/overrides/ostree-build-yocto it will be used
instead of the standard ostree-build-yocto.

This change allow us to use the python27 and git19 scls on rhel6
by having an override file that does something like:

 #!/bin/sh
scl enable python27 git19 "/PREFIX/lib/gnome-continuous/ostree-build-yocto $*"

Where PREFIX is where you installed gnome-continuous.