pass target arch to dpkg-buildpackage with a cmdline argument, not an ENV var

This was changed in dpkg-buildpackage, and the env variable was being silently
reset. This was broken a while back, and fixed here:

 https://anonscm.debian.org/cgit/dpkg/dpkg.git/commit/?id=e16a76d241f0c69e96bcfdc96b360cc7c82a9f48

The breakage was that specifying the target was impossible. The fix made it
possible, but changed the method, so this patch becomes necessary