bug#25414: gdk-pixbuf@2.34.0 does not build deterministically

[Danny Milosavljevic <dannym@scratchpost.org>]

Hi Ludo,

> Sounds like the right approach, would you like to test it?  :-)

In principle yes, but even without the patch I get (using current master):

$ ./pre-inst-env  guix challenge gdk-pixbuf --substitute-urls="https://mirror.hydra.gnu.org https://bayfront.guixsd.org"
...

Backtrace:
In unknown file:
   ?: 19 [apply-smob/1 #<catch-closure 233fde0>]
In ice-9/boot-9.scm:
  66: 18 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 17 [eval # #]
In ice-9/boot-9.scm:
2404: 16 [save-module-excursion #<procedure 235f900 at ice-9/boot-9.scm:4051:3 ()>]
4056: 15 [#<procedure 235f900 at ice-9/boot-9.scm:4051:3 ()>]
1727: 14 [%start-stack load-stack ...]
1732: 13 [#<procedure 2373b10 ()>]
In unknown file:
   ?: 12 [primitive-load "/x/home/dannym/src/guix-master/guix/scripts/guix"]
In guix/ui.scm:
1228: 11 [run-guix-command challenge "gdk-pixbuf" ...]
In ice-9/boot-9.scm:
 160: 10 [catch srfi-34 #<procedure 2a91220 at guix/ui.scm:432:2 ()> ...]
 160: 9 [catch system-error ...]
 160: 8 [catch system-error ...]
In guix/scripts/challenge.scm:
 238: 7 [#<procedure 2af1390 at guix/scripts/challenge.scm:224:6 ()>]
In guix/store.scm:
1223: 6 [run-with-store # ...]
In guix/scripts/challenge.scm:
 239: 5 [#<procedure 757dcf0 at guix/scripts/challenge.scm:239:15 (state)> #]
 123: 4 [#<procedure 88effc0 at guix/scripts/challenge.scm:123:2 (state)> #]
In srfi/srfi-1.scm:
 643: 3 [append-map #<procedure 74335a0 at guix/scripts/challenge.scm:125:46 (t-1367901)> ...]
 575: 2 [map #<procedure 74335a0 at guix/scripts/challenge.scm:125:46 (t-1367901)> ...]
In guix/scripts/substitute.scm:
 687: 1 [lookup-narinfos "\"https://mirror.hydra.gnu.org" #]
 659: 0 [fetch-narinfos "\"https://mirror.hydra.gnu.org" #]

guix/scripts/substitute.scm:659:16: In procedure fetch-narinfos:
guix/scripts/substitute.scm:659:16: In procedure struct_vtable: Wrong type argument in position 1 (expecting struct): #f