Tests: update the included `composer.phar` from v 2.2.2 to 2.2.18

[jrfnl]

Description

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

PR checklist

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING.md document.
• [ ] I have added tests to cover my changes.

[jrfnl]

Note: the tests will still fail on #236, but this is step 1 in fixing the current CI failure.

[desrosj]

On one hand, it's annoying to have to update the expected version every time ubuntu-latest changes. But on the other, it's a nice "Hey, something changed in the runner image" that could point to a reason for other strange behavior. Seems that Ubuntu images are updated about once a week.