substitute: 'http-multiple-get' processes each request only once.

substitute: 'http-multiple-get' processes each request only once.

Fixes <https://bugs.gnu.org/39090>.
Reported by Gábor Boskovits <boskovits@gmail.com>.
Fixes a regression introduced in 9e3f9ac3c00906f5bc647ea8398e4ed5a370614e.

* guix/scripts/substitute.scm (http-multiple-get): In the "Connection: close"
case, pass (drop requests (+ 1 processed)) to 'loop' as the remaining
REQUESTS value.

Previously, we would pass a list containing duplicates, and thus the
final result would also contain duplicates.  When sent to the daemon,
that list would lead to a daemon error:

  got unexpected path `/gnu/store/…' from substituter