bug#47458: Terrible UX upgrading Emacs in Guix

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

I just updated my Guix system, which included the Emacs update from 27.1
to 27.2.  After "guix package -m mhw-manifest.scm" finished running
(which takes a long time for me, since I don't use substitutes), and
before I even noticed that it had finished, my existing Emacs session
started misbehaving badly.

It failed to even open a plain text file in fundamental mode (a .drv
file) with an inscrutible error about 'arrayp'.  I tried to enable the
debugger with M-x toggle-debug-on-error, but then I started getting
errors about 'debug' not found.  (I neglected to record the exact
messages).

I tried running "emacs -Q", and the same errors happened there too.  I
tried running "emacs -Q" from my root shell on a Linux text terminal,
and the same errors happened there.

I resorted to using 'vi' (which thankfully I had, and still remember how
to use for basic editing) to revert the emacs update on my private
branch and to rebuild my user profiles.

Eventually, I realized what the problem was:

(1) My existing emacs session started failing because
    ~/.guix-profile/share/emacs/27.1 had disappeared out from under it.

(2) My newly launched emacs sessions were failing because my
    EMACSLOADPATH variable was still set to its old value, pointing at
    /home/mhw/.guix-profile/share/emacs/27.1/lisp, which no longer
    existed.

I'm not sure why I've never run into this problem before.  I'm also not
sure what can be done to make this better, but if anyone has ideas, that
would be good.  If a 7+ year Guix veteran developer gets bitten badly by
this, I doubt that less experienced users will be impressed.

Any ideas?

       Mark