Add composer to PATH (fix drush: command not found)

[kirkkala]

Description

Adding composer to PATH so drush can be ran on platformsh environment.

Motivation and Context

Created a platform.sh project with the template. drush command was not available in builds. Error like this appears on the build log

  Executing deploy hook for application drupal
    Created Drush configuration file: /app/.drush/drush.yml
    W: /app/drush/platformsh_deploy_drupal.sh: line 9: drush: command not found
    Drupal not installed. Skipping standard Drupal deploy steps

How Has This Been Tested?

After adding this change I was able to drush on platform.sh and the builds work.

Screenshots (if appropriate):

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following list, and put an x in all the boxes that apply. If you're unsure about what any of these mean, don't hesitate to ask. We're here to help!

• I have read the contribution guide
• I have created an issue following the issue guide
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[pjcdawkins]

The addition is good. (The DATABASE_URL stuff is not needed, but that's not relevant to this PR.)