bug#46967: Connection reuse for substitutes breaks with gzip

["Ludovic Courtès" <ludo@gnu.org>]

As of commit cdc688c5ee106b7ee2547e4abd74ab81d7702d3d, it seems that the
connection reuse code for substitutes breaks if you’re fetching gzip
substitutes.

I tweaked ‘narinfo-best-uri’ to always choose gzip, and here’s what I get:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix build vigra inkscape eigen pt-scotch 
accepted connection from pid 20839, user ludo
69.6 MB will be downloaded:
   /gnu/store/si36xnajxjnx7v6ywd7qyhyv8ar9aymn-vigra-1.11.1
   /gnu/store/0fcjk788gj7k6gsf3zsx8j8lyw6w896b-inkscape-1.0.2
substituting /gnu/store/0fcjk788gj7k6gsf3zsx8j8lyw6w896b-inkscape-1.0.2...
downloading from https://ci.guix.gnu.org/nar/gzip/0fcjk788gj7k6gsf3zsx8j8lyw6w896b-inkscape-1.0.2 ...
 inkscape-1.0.2  38.6MiB                                                    16.8MiB/s 00:02 [##################] 100.0%

substituting /gnu/store/si36xnajxjnx7v6ywd7qyhyv8ar9aymn-vigra-1.11.1...
Backtrace:
In guix/ui.scm:
  2164:12 19 (run-guix-command _ . _)
In guix/scripts/substitute.scm:
    664:2 18 (guix-substitute . _)
In unknown file:
          17 (with-continuation-barrier #<procedure thunk ()>)
In ice-9/boot-9.scm:
  1736:10 16 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
          15 (apply-smob/0 #<thunk 7fa67ce511c0>)
In ice-9/boot-9.scm:
  1736:10 14 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
  1736:10 13 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
  1731:15 12 (with-exception-handler #<procedure 7fa67c302390 at ice-9/boot-9.scm:1815:7 (exn)> _ #:unwind? _ # _)
In guix/scripts/substitute.scm:
   713:17 11 (_)
    414:7 10 (process-substitution _ "/gnu/store/si36xnajxjnx7v6ywd7qyhyv8ar9aymn-vigra-1.11.1" #:cache-urls _ #:acl …)
In ice-9/boot-9.scm:
  1736:10  9 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In guix/scripts/substitute.scm:
    423:9  8 (_)
In ice-9/boot-9.scm:
  1731:15  7 (with-exception-handler #<procedure 7fa67c42edb0 at ice-9/boot-9.scm:1815:7 (exn)> _ #:unwind? _ # _)
  1669:16  6 (raise-exception _ #:continuable? _)
  1667:16  5 (raise-exception _ #:continuable? _)
  1669:16  4 (raise-exception _ #:continuable? _)
  1764:13  3 (_ #<&compound-exception components: (#<&error> #<&irritants irritants: (http-version "_'$Àçv\x8b\x8f7…>)
  1669:16  2 (raise-exception _ #:continuable? _)
  1667:16  1 (raise-exception _ #:continuable? _)
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
Bad http-version header component: _'$Àçv‹7$H?

Backtrace:
In ice-9/boot-9.scm:
  1736:10  4 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
           3 (apply-smob/0 #<thunk 7fa67e9cb520>)
In ice-9/boot-9.scm:
    718:2  2 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8  1 (_ #(#(#<directory (guile-user) 7fa67e9cec80>)))
In guix/ui.scm:
  2164:12  0 (run-guix-command _ . _)

guix/ui.scm:2164:12: In procedure run-guix-command:
Bad http-version header component: _'$Àçv‹7$H?

substitution of /gnu/store/si36xnajxjnx7v6ywd7qyhyv8ar9aymn-vigra-1.11.1 failed
guix build: error: some substitutes for the outputs of derivation `/gnu/store/k32m5dcf31psvx66n95icxbvyimrjrk5-vigra-1.11.1.drv' failed (usually happens due to networking issues); try `--fallback' to build derivation from source 
$ wget -qO - https://ci.guix.gnu.org/nar/gzip/si36xnajxjnx7v6ywd7qyhyv8ar9aymn-vigra-1.11.1 | gunzip > /dev/null
$ echo $?
0--8<---------------cut here---------------end--------------->8---

Ludo’.