bug#36375: ‘guix package’ should lock the profile

[Ludovic Courtès]

The article at
<https://distrowatch.com/weekly.php?issue=20190624#guixsd> mentions
things like:

  For instance, after installing Icecat, I installed a few other desktop
  programs and then found Icecat had disappeared from my path again.

It’s likely that the person ran several ‘guix package’ commands in
parallel, and that one undoed the effects of the other.

Julien suggested that ‘guix package’ could grab a lock file, and I guess
it could simply error out when the lock is already taken.

Ludo’.