build-image: use the full path to losetup

Sync the filesystem and use the full losetup path to make the loop
device detach more reliable.
Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>

build-image

@@ -233,7 +233,8 @@ if [ "${arch}" == "arm64" ]; then
       # Merge boot image inside the main image
       loop=$(setup_loop "${mainimg}")
       sudo dd if="files/uboot-${uboot_type}/${uboot_type}-boot.img" of="${loop}" bs=1024 seek=2
-      sudo losetup -d "${loop}"
+      sudo sync
+      sudo /sbin/losetup -d "${loop}"
     fi
 fi