Unable to install PHPUnit on older containers

[NathanielWalmsley]

Description
Some security issues require us to use old versions of the site, going back as far as 10. This version of Totara requires PHP 5.6. The installunit command requires composer to be installed on the container, but the standard version of composer does not support PHP 5.6, and calling composer self-update --2.2 doesn't work.

We still need to support older versions of the site for security patches, so we'll still need to be able to support older versions of PHP.

To Reproduce

1. Check out an older version of the site (e.g. T10)
2. Bring up a PHP 5.6 container
3. Call installunit inside the container. You will see:
   Composer 2.3.0 dropped support for PHP <7.2.5 and you are running 5.6.40, please upgrade PHP or use Composer 2.2 LTS via "composer self-update --2.2". Aborting.
4. Attempt to update composer using composer self-update --2.2. This will fail because the composer command is not found.

Expected behavior
The container should be able to install PHPUnit without requiring composer.