bug#36275: warn-about-diskspace fails with a stacktrace

[Björn Höfling]

A successful build on a system with little disk-space ends with:

$ ./pre-inst-env  guix system disk-image system.scm
[..]
/gnu/store/a6p78ff1pjd6rdmf58066aidxlmiaqlm-disk-image
Backtrace:
           8 (apply-smob/1 #<catch-closure 61f1a0>)
In ice-9/boot-9.scm:
    705:2  7 (call-with-prompt _ _ #<procedure default-prompt-handle?>)
In ice-9/eval.scm:
    619:8  6 (_ #(#(#<directory (guile-user) 6dd140>)))
In guix/ui.scm:
  1620:12  5 (run-guix-command _ . _)
In ice-9/boot-9.scm:
    829:9  4 (catch _ _ #<procedure 7f357601bc80 at guix/ui.scm:587?> ?)
    829:9  3 (catch _ _ #<procedure 7f357601bc98 at guix/ui.scm:710?> ?)
In guix/scripts/system.scm:
   1325:8  2 (_)
In guix/status.scm:
    768:4  1 (call-with-status-report _ _)
In guix/scripts.scm:
   201:10  0 (warn-about-disk-space _ #:threshold _)

guix/scripts.scm:201:10: In procedure warn-about-disk-space:
In procedure private-lookup: No variable bound to print-diagnostic-prefix in 
module (guix ui)

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