Update composer path on findComposer

[peter279k]

Reopen PR #85.

Changed log

• Update the composer execution path on NewCommand::findComposer command.

Actually, the original php compoer.phar string result is not correct.

And this will not specify the current path to let PHP binary know where the composer.phar can execute.

It should use the current composer path to tell PHP binary to execute composer.phar.

Instruction

Environment

• Ubuntu 18.04
• composer version is 1.8.4.

Here are steps about reproducing the issue.

• Firstly, we use the composer require laravel/installer command.
• Then using the vendor/bin/laravel new blog to create the new Laravel project.
• It will get the error message in following image during building the Laravel project.