build: Don't compile schemas if none are installed
If the backend isn't built, no schemas are installed, but we'd still try to compile the schemas.
Running custom install script '/usr/bin/glib-compile-schemas /app/share/glib-2.0/schemas'
Error opening directory “/app/share/glib-2.0/schemas”: No such file or directory
FAILED: install script '/usr/bin/glib-compile-schemas /app/share/glib-2.0/schemas' exit code 1, stopped
Fixes: 49daccd9 ("build: Use gnome.post_install() function")
Broken pipeline: https://gitlab.gnome.org/GNOME/gnome-online-accounts/-/jobs/2275539
Fixed pipeline: https://gitlab.gnome.org/GNOME/gnome-online-accounts/-/jobs/2275566
Edited by Bastien Nocera