Issue #3480807 by jwilson3: Switch to ddev-drupal-contrib fork for themes

ddev add-on get https://github.com/jameswilson/ddev-drupal-contrib/tarball/111-env-var-project-path

Hopefully this is a temporary change while https://github.com/ddev/ddev-drupal-contrib/issues/111 is being reviewed for inclusion.

Closes #3480807