When a user adds a package to the guix config.scm and installs it with
guix system reconfigure the application shows up in the Gnome
application menu but does not have an icon until the user logs out and
back in.

Additionally, adding packages to a Guix Home definition and installing
it either with guix system reconfigure via the guix-home-service-type
service or explicitly with guix home reconfigure, the application does
not show up at all in the Gnome application menu until the user logs
out and back in.

Note, when a user installs an application to their profile via guix
install, there is no issue, the application appears in the Gnome
application menu and it has a valid icon without needing to log out
and back in.

I've attached three screenshots to illustrate the issue with 'vlc'
added as an (operating-system) package and 'gimp' added as a
(home-environment) package:

1-before-reconfigure.png - This is the initial state with their 'vlc'
nor 'gimp' installed.

2-after-reconfigure.png - This is after guix system reconfigure is run.
- 'vlc' is installed and appears in the application menu but does not
have the correct icon.
- 'gimp' is installed but does not appear in the application menu.

3-after-relogin.png - This is after the user logs out and back in,
- 'vlc' now has the appropriate icon.
- 'gimp' now appears in the menu (and has an appropriate icon).

Additional notes: this happens regardless if a package was added
directly or with the specifications->packages helper, in either system
or home.