bug#31841: ./pre-inst-env guix system no longer works without ~/.config/guix

[Mark H Weaver <mhw@netris.org>]

Since updating to current master (commit b494bbe46), 'guix system' no
longer works for me:

--8<---------------cut here---------------start------------->8---
mhw@jojen ~/guix$ ./pre-inst-env guix system build -n /etc/config.scm
guix: system: command not found
Try `guix --help' for more information.
--8<---------------cut here---------------end--------------->8---

However, 'guix package' and 'guix build' still work:

--8<---------------cut here---------------start------------->8---
mhw@jojen ~/guix$ ./pre-inst-env guix package -A icecat
icecat	52.6.0-gnu1	out	gnu/packages/gnuzilla.scm:412:2
mhw@jojen ~/guix$ ./pre-inst-env guix build e2fsprogs
/gnu/store/0149ikffk8y86yns9f338k2s93nv1vb1-e2fsprogs-1.43.6
--8<---------------cut here---------------end--------------->8---

Note that I haven't used 'guix pull' in years, and I don't want to use
it.  My ~/.config/guix/ is empty.  I run 'guix' exclusively from git
checkouts using ./pre-inst-env.

Is this still a supported mode of operation?
It is feasible for me to continue in this way?

       Mark