check for glib-compile-schemas presence before calling from the ubuntu hook

This makes the hook ok to use cross-flavor.

We could also move glib-compile-schemas to a separate hook, to ensure we never
silently fail because glib-compile-schemas is broken/missing.

debian/changelog

@@ -2,6 +2,8 @@ livecd-rootfs (24.04.15) UNRELEASED; urgency=medium
 
   * live-build/ubuntu/hooks/020-ubuntu-live.chroot_early: apply to any
     layer ending in .live so this can be reused across flavors
+  * live-build/ubuntu/hooks/020-ubuntu-live.chroot_early: check for
+    glib-compile-schemas presence before calling
 
  -- Steve Langasek <steve.langasek@ubuntu.com>  Wed, 20 Dec 2023 19:45:34 -0800
 

live-build/ubuntu/hooks/020-ubuntu-live.chroot_early

@@ -14,4 +14,6 @@ cat <<EOF > /etc/initramfs-tools/conf.d/default-layer.conf
 LAYERFS_PATH=${PASS}.squashfs
 EOF
 
-glib-compile-schemas /usr/share/glib-2.0/schemas/
+if which glib-compile-schemas >/dev/null 2>&1; then
+	glib-compile-schemas /usr/share/glib-2.0/schemas/
+fi