Otherwise the build will use 'python' for generated scripts. Since
we depends_dep=python3, we want all generated scripts on python3 or
the we end up not pulling right interpreted. Fixes errors on some
builders:
/usr/bin/glib-genmarshal --prefix=gq_marshal ./gq-marshal.list --header >gq-marshal.h
env: can't execute '/usr/bin/python': No such file or directory