t2u builder vm home directory needs public half of the signing key

By perpetrating extreme violence I got as far as this (job 40):

+ gpg --detach-sign --armor -u 'debian-tag2upload@lists.debian.org' tag.tmp
.....
 + git -c -cdgit-distro.debian/push.git-check=false -c '-cdgit-distro.debian/push.ssh=; exit 0; ' verify-tag f80ac5aebb4903b2eca23f4aa1be88f59c69f147
gpg: Fatal: can't create directory '/nonexistent/.gnupg': No such file or directory
dgit (build host): failed command: git -c -cdgit-distro.debian/push.git-check=false -c '-cdgit-distro.debian/push.ssh=; exit 0; ' verify-tag f80ac5aebb4903b2eca23f4aa1be88f59c69f147

I think this is blocked on the home directory existing which is also needed for #52 (closed).

(The -c options are garbage. This is some unrelated bug with dgit remote argument processing, which I will take on. I don't think it's currently in our way for t2u except that it interfereres a bit with the extreme violence I mentioned.)