bug#56076: guix shell -P behavior is surprising with persistent home

[Ethan Blanton]

I am using guix shell to create an isolated container, but using a
persistent home directory to preserve configuration and state for the
program in the container.  Specifically, I am using (lightly
simplified; note that the user is elb and the manifest contains ONLY
syncthing):

CONTAINER_HOME=/path/to/persistent/home
MANIFEST=/path/to/manifest.scm

guix shell --container --network --no-cwd -P      \
           --share=$CONTAINER_HOME=/home/elb --   \
           /bin/sh -c 'SSL_CERT_DIR="$HOME/.guix-profile/etc/ssl/certs" 
syncthing'

Because syncthing requires a certificate store, the easiest way to
configure that seemed to be through the profile /etc dir, which meant
using -P.  The above command works, and works correctly, but only the
first time the profile is started.

On subsequent starts, guix complains that:

guix shell: error: cannot link profile: '/home/elb/.guix-profile' already 
exists within container

This is easily worked around by removing .guix-profile from the
--share-bound home directory before invoking guix shell.

It is not clear to me that this is a bug, but it was surprising.  It
was also surprising that there was not an obvious way to simply
declare a persistent home directory for a container, although now that
I understand the `guix shell` command better, I find this less
surprising than I did when I was first exploring.

Ethan