bug#68811: build hash inconsistency

[Zacchaeus Scheffer]

Hi all,

tl;dr I run the following command on two aarch64-linux machines and get
two different hashes for the 'qutebrowser' package:

guix time-machine --commit=deeb7d1f53d7ddfa977b3eadd760312bbd0a2509 -- build 
qutebrowser --dry-run

Both machines use only the main guix repository, and guix describe gives
the same output (except generation number and date, which is fine).
Coming from aarch64, building is incredibly expensive.  If the build
hash doesn't match, then (I believe) there is no hope that my machine
will find the packages on a substitute server.  To get around this
issue, I built my guix home once, guix copy'd the store items, and
manually added a symlink in
/var/guix/profiles/per-user/USER/guix-home-N-link to point to the
foreign guix home build.  I couldn't find this issue elsewhere in the
issues, but my "hashes don't match" problem is pretty vague.

Is this an expected problem?  Is this a novel problem?  Am I
misunderstanding guix time-machine (which seems like it should produce
an identical store item)?

-Zacchae