can't substitute gtk+ from a guix-publish server, it fails with:
GET /nar/1dlz1am0qmj1579f5p6j5cvfx9l2aw50-gtk+-3.18.2
In web/server/http.scm:
117:27 1 (http-read #<<http-server> socket: #<input-output: so...>)
In unknown file:
0 (peek-char #<input-output: socket 13>)
ERROR: In procedure fport_fill_input: Connection reset by peer
