In "nix-env -qas", we don't need the substitute info, we just need to
know if it exists. This can be done using a HTTP HEAD request, which
saves bandwidth.
Note however that curl currently has a bug that prevents it from
reusing HTTP connections if HEAD requests return a 404:
https://sourceforge.net/tracker/?func=detail&aid=3542731&group_id=976&atid=100976
Without the patch attached to the issue, using HEAD is actually quite
a bit slower than GET.