Fixed preferSource Composer param

[corpsee]

Contribution Type: bugfix Link to Intent to Implement: Link to Bug:

This pull request affects the following areas:

• [ ] Front-End
• [ ] Builder
• [x] Build Plugins

In raising this pull request, I confirm the following (please check boxes):

• [x] I have read and understood the contributing guidelines?
• [x] I have checked that another pull request for this purpose does not exist.
• [x] I have considered, and confirmed that this submission will be valuable to others.
• [ ] I have created or updated the relevant documentation for this change on the PHPCI Wiki.
• [ ] Do the PHPCI tests pass?

Detailed description of change:

Fixed preferSource Composer param