Tests: update the included composer.phar from v 2.2.2 to 2.2.18 (#237)

Looks like the default PHP version on `ubuntu-latest` has been updated to PHP 8.1.11, which means that the `expect` tests running with the included `composer.phar` will error out on a fatal `PHP Fatal error:  Uncaught TypeError: array_merge(): Argument #1 must be of type array, bool given in phar:///home/runner/work/composer-install/composer-install/tests/fixtures/composer.phar/src/Composer/Config.php:191` error.

Updating the included `composer.phar` to a more recent version fixes this.

Alternatively, the file could be removed and downloaded (or updated) on the fly from within CI ?

Refs:
* https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2204-Readme.md

Co-authored-by: jrfnl <jrfnl@users.noreply.github.com>