nss@3.99 is really hard to build, it's so hard and so important that
data.guix.gnu.org is still after two days trying to process [1]. I say
so important because you have to build nss@3.99 to compute the channel
instance derivations for Guix.

1: https://data.guix.gnu.org/revision/72308f262c910977e40c2c9f350dc563c0a8437a

Looking at the next revision which has been processed [2], it's been
built on riscv64-linux as the testsuite is disabled, and it has also
built on aarch64-linux, but there's no successful build for any other
architecture.

2: https://data.guix.gnu.org/revision/9f183c3627a006e8fd3bb9708448bc05a6204e6d/package/nss/3.99.0?locale=en_US.UTF-8

I think there's two issues here, was this spotted before merging, and
what if anything can be done about this now. Where there's not a
substitute available for nss@3.99, this will affect guix pull/guix
time-machine, e.g.

  → guix time-machine --commit=72308f262c910977e40c2c9f350dc563c0a8437a -- describe
  Updating channel 'guix' from Git repository at 'https://git.savannah.gnu.org/git/guix.git'...
  substitute: updating substitutes from 'https://bordeaux.guix.gnu.org'... 100.0%
  substitute: updating substitutes from 'https://bordeaux.guix.gnu.org'... 100.0%
  substitute: updating substitutes from 'https://bordeaux.guix.gnu.org'... 100.0%
   nss-3.99.tar.xz  55.2MiB                                                                                             13.7MiB/s 00:04 ▕██████████████████▏ 100.0%
  building /gnu/store/8379qa0y6s7ssjr8gplm5fyw9r5pnxhn-nss-3.99.0.drv...