The environment variables can't be passed through an env: argument
yet (meson#2723), use 'env' as suggested in:
https://github.com/mesonbuild/meson/issues/2723#issuecomment-348630957
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>