custom_scripts: ensure usr-is-merged does not fail the migration test

The new usr-is-merged metapackage intentionally fails to install on systems that are not merged-usr, unless a specific override flag is passed.

usrmerge does a live-upgrade of the chroot, which piuparts does not expect.

Use the skip flag, so that migration is not blocked. Debootstrap will start using this flag once the package is available in testing.