"Command not found" when using `home-mcron-service`

[Tanguy LE CARROUR <tanguy@bioneland.org>]

Dear Guix,

I had stop using `home-mcron-service` some times ago, for I had
a problem with my home shepherd. Now that this problem seems to be
fixed 🤞, I've tried to re-enable `home-mcron-service` and, while it runs,
it cannot find the commands to execute!?

I used to have this config:

```scheme
(service home-mcron-service-type
        (home-mcron-configuration
         (jobs (list
                 #~(job '(next-minute (range 0 60 5)) "notmuch new")
                 #~(job '(next-hour) "vdirsyncer sync")))))
```

But I now have `notmuch new: sh: line 1: notmuch: command not found` in
my logs.

So, I changed it to:

```scheme
(gnu packages dav)  ; vdirsyncer
(gnu packages mail)  ; notmuch
; […]
(service home-mcron-service-type
        (home-mcron-configuration
         (jobs (list
                 #~(job '(next-minute (range 0 60 5)) (string-append #$notmuch "/bin/notmuch new"))
                 #~(job '(next-hour) (string-append #$vdirsyncer "/bin/vdirsyncer sync"))))))
```

And now Mcron finds `notmuch` (and `vdirsyncer`), but… Notmuch has hooks
that depend on other commands and I now have the following error message
in my logs:

```
2024-01-15 10:20:00 6198 /gnu/store/[…]-notmuch-0.37/bin/notmuch new: […]/.config/notmuch/default/hooks/pre-new: line 3: mbsync: command not found
```

`mbsync` is in my profile, but Mcron cannot find it.
Do I have to declare all the required packages… somehow?! 🤔

Any help welcome!

Regards,

-- 
Tanguy