bug#50916: Experimental `guix container PID program args` not working as expected

[kiasoc5]

This bug still persists and effectively bypasses container sandboxing:

1. Start a container shell.

$ guix shell --container

2. Exec a shell inside the container. Here it is a foreign distro's
bash, and assume that there is only 1 container running.

$ guix container exec $(pgrep --full 'guix shell --container') /bin/bash

3. Observe that path is not empty.

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/bin

This shell can access commands outside the container, such as kill
(part of util-linux).

4. Kill the container from the shell inside the container.

[env] $ kill -9 $(pgrep --full 'guix shell --container')
[env] $ echo $?
0

The shell still persists in the container, even though the container
itself is killed.

The problem is that in guix/scripts/container/exec.scm, execlp is used
to launch program with args. info guile says this about execlp:

  Similar to ‘execl’, however if FILENAME does not contain a slash then
  the file to execute will be located by searching the directories
  listed in the ‘PATH’ environment variable.

As seen above $PATH is set to a subset of the host system's PATH, not
the PATH inside the guix container.