This reverts commit bd974458.

I didn't realize there is already a helper make-gnome-colors.
Helpers need to have the source package's name, in this case
gnome-colors-common is a binary package prooduced by source
package gnome-colors.

I'm reverting this, and manually merging the code from both helpers.

shim-signed is not avaliable in Trisquel

- Avoid installing packages and using system files

Fix leny2010/package-helpers#7 Bug 11795

Fix leny2010/package-helpers#8 Bug 11794

Fix leny2010/package-helpers#6 Bug 11793

Fix leny2010/package-helpers#7 Bug 10713

As per leny2010/package-helpers#5

It wasn't a build-dep, and the only contents were packages not avaliable in trisquel.

Fixes #12785