Subject: guix can't mount mountpoint path, and the home path.

From: bbb ee <[email-masked]>
Date: Mon, 21 Nov 2022 21:31:58 +0100

The new tests show guix can mount all path except mountpoint path, and the
home path.
```
dev_1@dev_1 /mnt/recoverData$ mountpoint . ; guix shell --container
coreutils -- echo Elmo
. is a mountpoint
guix shell: error: mount: mount "/mnt/recoverData" on
"/tmp/guix-directory.Fu8aV9//mnt/recoverData": Invalid argument

dev_1@dev_1 /mnt/recoverData/borg_restore$ mountpoint . ; guix shell
--container coreutils -- echo Elmo
. is not a mountpoint
Elmo

dev_1@dev_1 ~$ mountpoint . ; guix shell --container coreutils -- echo Elmo
. is not a mountpoint
guix shell: error: mount: mount "/home/dev_1" on
"/tmp/guix-directory.shXxIg//home/dev_1": Invalid argument

dev_1@dev_1 ~/Downloads$ mountpoint . ; guix shell --container coreutils --
echo Elmo
. is not a mountpoint
Elmo
```