bug#70841: Upgrade-Shepherd-Services Error when adding wayland support to gdm

[not emma]

The following added to my system config:
(modify-service %desktop-services
   (gdm-service-type config =>
     (gdm-configuration
       (inherit config)
       (wayland? #t)))))

Results in the below error:

guix system: error: error parsing derivation
`/gnu/store/4xz1998qw8niax3pi0lsbwxfl7dfrpil-upgrade-shepherd-services.drv':
expected string `Derive(['

(apologies for any formatting issues i currently can't access any of my guix system due to this, so im typing from my phone)

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