fix(manager/composer): respect the PHP override in composer.json (#13657)

* fix(composer): respect the PHP override in composer.json

Signed-off-by: Gary Lockett <gary@creativecow.uk>

* chore(docs): document new configuration

Signed-off-by: Gary Lockett <gary@creativecow.uk>

* Apply suggestions from code review

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

* fix(tests): add missing composer version to expected output

Signed-off-by: Gary Lockett <gary@creativecow.uk>

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>
Co-authored-by: Rhys Arkins <rhys@arkins.net>