If the PHP binary is not in the PHP interpreter's PATH, Composer invocations fail

Problem/Motivation

Well, this is awkward, but in Automatic Updates, we have a situation where the PHP interpreter is trying to execute another, external command which uses the PHP interpreter: Composer.

Composer's executable, like most PHP-based executables, begins with /usr/bin/env php. Usually that's fine...but if php is not in the PATH of the environment where Composer is being invoked, it will fail. That pretty much completely breaks this module.

Steps to reproduce

Try to do anything with Automatic Updates in a situation where the PHP interpreter does not have the php binary in its PATH.

Proposed resolution

The Composer runner service (provided by Composer Stager, potentially overridable by us) should probably prefix the Composer command invocations with the value of the PHP_BINARY constant. It's unclear how that will work if PHP_SAPI is something other than cli, but it definitely improves the chances of this working.

Remaining tasks

Do it, and add some sort of unit test which proves that Composer is invoked by the PHP interpreter directly.