bug#50441: Wrong build directory number shown in environment-variables file

[Christine Lemmer-Webber]

I was doing guix build with "--keep-missing" and looking around in a few
of the output directories.  I was peeking in:

  /tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-4

But wait... the "environment-variables" file says:

#+BEGIN_SRC sh
export TEMP=\
"/tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-0"
export TEMPDIR=\
"/tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-0"
export TMP=\
"/tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-0"
export TMPDIR=\
"/tmp/guix-build-u-boot-mnt-reform2-2021.06.drv-0"
#+END_SRC

Now wait a minute.  Look at that last number.  What the hell is going on
here?  Is this a bug in Guix?  Why is it pointing at -0 in the -4 build
directory?

Does this point at a more serious issue?  What's going on?