* More haste, less speed:

- Perform in-place edit.
  - Allow variable expansion.

live-build/ubuntu-core/hooks/03-boot_with_systemd.chroot

@@ -3,7 +3,7 @@
 # Boot using systemd and disable quiet boot
 # to see what is happening.
 systemd=/lib/systemd/systemd
-sed \
+sed -i \
-    -e 's,^\([ 	]*GRUB_CMDLINE_LINUX\)=""$,\1="init=$systemd",g' \
+    -e "s,^\([ 	]*GRUB_CMDLINE_LINUX\)=\"\"$,\1=\"init=$systemd\",g" \
-    -e 's,^\([ 	]*GRUB_CMDLINE_LINUX_DEFAULT\)="quiet splash",\1="",g' \
+    -e "s,^\([ 	]*GRUB_CMDLINE_LINUX_DEFAULT\)=\"quiet splash\",\1=\"\",g" \
     /etc/default/grub