Install image with custom services

[Jack Hill]

Hello Guix,

I'm trying to create an installation image that uses Network Manager to 
configure networking (For use at a site with WPA-Enterprise networks). I 
came up with the following:

```scheme
(define-module (jackhill system install)
  #:use-module ((gnu packages gnome) #:select (network-manager) #:prefix 
package:)
  #:use-module (gnu services)
  #:use-module (gnu services networking)
  #:use-module (gnu system)
  #:use-module (gnu system install)
  #:use-module (srfi srfi-1)
  #:export (my-installation-os))

(define my-installation-os
  (operating-system
    (inherit installation-os)
    (packages (cons package:network-manager
                     (operating-system-packages installation-os)))
    (services (cons (service network-manager-service-type)
                (remove (lambda (service)
                          (eq? (service-kind service) connman-service-type))
                        (operating-system-services installation-os))))))

my-installation-os
```

However, passing that to guix system disk-image results in:

```
guix system: error: more than one target service of type 'shepherd-root'
```

Thinking that I had done something wrong with removing the connman-service 
and adding the network-manager-service, I created a more minimal operating 
system definition:

```scheme
(define my-installation-os
  (operating-system
    (inherit installation-os)
    (packages (cons package:network-manager
                     (operating-system-packages installation-os)))
    (services (operating-system-services installation-os))))
```

To my surprise, this also fails with

```
guix system: error: more than one target service of type 'account'
```

However,

```scheme
(define my-installation-os
  (operating-system
    (inherit installation-os)
    (packages (cons package:network-manager
                     (operating-system-packages installation-os)))))
```

sucseeds.

Should these last two operating system definitions not be equivalent? 
What's going on here?

Thanks,
Jack