Permission denied then running guix shell -C on Ubuntu

[Gabriel Pickl]

Hi everyone :)

I've recently started using GUIX on Ubuntu 24.04 (Installed via the 
install script), and have run into a bit of a problem.

When running something like `guix shell -C guile` (the package list 
doesn't matter) I get the following error message:

```
guix shell: error: mount: mount "none" on "/tmp/guix-directory.xwKsHW": 
Permission denied
```

`dmesg` doesn't show any messages during the run.

Turning AppArmor off changes the error:

```
guix shell: error: clone: 2114060305: Permission denied
```

And also causes the following dmesg line to be printed (I thought I had 
disabled AppArmor... huh)

```
audit: type=1400 audit(1714930774.939:64): apparmor="DENIED" 
operation="userns_create" class="namespace" info="Userns create 
restricted - failed to find unprivileged_userns profile" error=-13 
profile="unconfined" pid=5486 comm="guix" requested="userns_create" 
denied="userns_create" target="unprivileged_userns"
```

I found some bug reports that might be related, but I don't know enough 
about GUIX or AppArmor (mentioned below) to extract anything useful from 
them

 * https://issues.guix.gnu.org/61690
 * https://issues.guix.gnu.org/46292
 * https://www.mail-archive.com/ubuntu-bugs@lists.ubuntu.com/msg6057761.html