See https://travis-ci.org/wp-cli/package-command/builds/209899744

01. $ php -dphar.readonly=0 'features/bootstrap/../../utils/make-phar.php' '/tmp/wp-cli-test-run-58c317feeaf023.95759079/wp-cli-build-58c317feeaff24.74413409.phar' --version='0.23.0' && chmod +x '/tmp/wp-cli-test-run-58c317feeaf023.95759079/wp-cli-build-58c317feeaff24.74413409.phar'
    Could not open input file: features/bootstrap/../../utils/make-phar.php

Which option makes more sense?

1. Include utils/make-phar.php as a one-off in this repo.
2. Add utils/make-phar.php to wp scaffold package-tests so that the Phar creation step is available to every WP-CLI command.
3. Add utils/make-phar.php to the generated WP-CLI Phar file, so that it's available

I'm leaning towards 3 at this point, so there's always one source of truth for Phar generation.