bug#52011: pkexec: PATH environment variable

[Hamzeh Nasajpour]

The `PATH` environment variable is hard-code here: 
https://github.com/freedesktop/polkit/blob/master/src/programs/pkexec.c#L882-L886

We don't have any executable in these paths in guix:
```
/usr/sbin:/usr/bin:/sbin:/bin:/root/bin
``` 

Replicate the issue:
1. Run the `pkexec`
2. Enter your password
3. run `echo $PATH` in the opened terminal
4. You will see this path: `/usr/sbin:/usr/bin:/sbin:/bin:/root/bin`
5. You can't run most of the commands. (`ls`, `passwd`, `chpasswd` and so on.)

Expected Behavior:
Running all of the commands without any error.

Isn't it? Should not we patch the `PATH` environment variable in `pkexec` 
source codes? Either way, some applications like `lxqt-admin-user` and 
`lxqt-admin-time` has an issue and they can't run the commands via `pkexec`. I 
get this error when I want to change user password via `lxqt-admin-user`. It's 
using `pkexec` to change password.

```
/run/current-system/profile/bin/lxqt-admin-user-helper: line 7: exec: passwd: 
not found
``` 

--

Hamzeh Nasajpour
PantherX Team