Hi ng0,

I've tried using your hydra.gnu.org mirror, and unfortunately it fails
for me as follows:

--8<---------------cut here---------------start------------->8---
[1] marusich@garuda.local:~
$ guix build --substitute-urls=https://guix-mirror.infotropique.org hello
substitute: Backtrace:
substitute:            1 (primitive-load "/gnu/store/d0xkr3d3zyqrnsh31vp0qdavp3a…")
substitute: In guix/ui.scm:
substitute:   1442:12  0 (run-guix-command _ . _)
substitute: 
substitute: guix/ui.scm:1442:12: In procedure run-guix-command:
substitute: guix/ui.scm:1442:12: In procedure put-string: Wrong type argument in position 1 (expecting open output port): #<closed: string 29e4540>
guix build: error: build failed: substituter `substitute' died unexpectedly
[1] marusich@garuda.local:~
$ guix build --substitute-urls='https://hydra.gnu.org https://guix-mirror.infotropique.org' hello
substitute: updating list of substitutes from 'https://hydra.gnu.org'... 100.0%
substitute: updating list of substitutes from 'https://hydra.gnu.org'... 100.0%
guix build: error: build failed: got unexpected path `following redirection to `https://c.n0.is/nix-cache-info'...' from substituter
[1] marusich@garuda.local:~
$ guix build --substitute-urls=https://hydra.gnu.org hello
The following derivation will be built:
   /gnu/store/3lk7pn0q4bnh33ldi1ppd6x2hlkrc0rk-hello-2.10.drv
0.0 MB will be downloaded:
   /gnu/store/wf65hjwqwpz4wllasn63zysi5irql2sx-hello-2.10
@ substituter-started /gnu/store/wf65hjwqwpz4wllasn63zysi5irql2sx-hello-2.10 /gnu/store/d0xkr3d3zyqrnsh31vp0qdavp3anh6ml-guix-0.13.0-10.0b4c385/libexec/guix/substitute
Downloading https://hydra.gnu.org/guix/nar/gzip/wf65hjwqwpz4wllasn63zysi5irql2sx-hello-2.10...
 hello-2.10  48KiB                                                                             463KiB/s 00:00 [####################] 100.0%

@ substituter-succeeded /gnu/store/wf65hjwqwpz4wllasn63zysi5irql2sx-hello-2.10
@ build-started /gnu/store/3lk7pn0q4bnh33ldi1ppd6x2hlkrc0rk-hello-2.10.drv - x86_64-linux /var/log/guix/drvs/3l//k7pn0q4bnh33ldi1ppd6x2hlkrc0rk-hello-2.10.drv.bz2
grafting '/gnu/store/wf65hjwqwpz4wllasn63zysi5irql2sx-hello-2.10' -> '/gnu/store/pa4w02b89d6sq33840dxfl5vbqbwz5iy-hello-2.10'...
@ build-succeeded /gnu/store/3lk7pn0q4bnh33ldi1ppd6x2hlkrc0rk-hello-2.10.drv -
/gnu/store/pa4w02b89d6sq33840dxfl5vbqbwz5iy-hello-2.10
[0] marusich@garuda.local:~
$ 
--8<---------------cut here---------------end--------------->8---

Does it work for you?

-- 
Chris