Substitute not being used

[Saku Laesvuori]

I'm trying to install telegram-desktop and I *think* guix should be able
to substitute it, but instead it tries to build it. (which then
consistently fails, which is why I started looking into it)

$ guix describe
Generation 11   Jul 25 2022 10:15:41    (current)
  guix 2367469
    repository URL: https://git.savannah.gnu.org/git/guix.git
    commit: 236746900f062b34c857c4aab21709bcf1448ac7

There is a build for telegram-desktop for x86_64-linux
(https://ci.guix.gnu.org/build/1119886/details) that is part of an
evaluation on the same commit that my guix is on.

One confusing thing I've noticed while looking into this is that when
guix starts building telegram-desktop it says it's building

/gnu/store/aqhqhza58lzkvzxrspzbf8v9ssqvkxri-telegram-desktop-2.9.3.drv

but the derivation on ci.guix.gnu.org is

/gnu/store/gz50dz1rfaik7s33d22r2lmyzwd7f7bm-telegram-desktop-2.9.3.drv

Can someone explain what's happening here and what I have misunderstood?

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