Sync script does not yet support multiple versions of PHP

[gregharvey]

We've seen build output like this on sync attempts:

4:54:57 [[email protected]] out: Error: Cannot use lexical variable $value as a parameter name in
14:54:57 [[email protected]] out: /var/www/XXXXXXX_test_build_765/docroot/sites/all/modules/contrib/webform/components/select.inc,
14:54:57 [[email protected]] out: line 765
14:54:57 [[email protected]] out: 

This is a direct result of PHP 7.x being used to sync a site that needs PHP 5.x. We need to be able to set PHP version like we can in the build scripts.