The P4 Plugin does not inject the P4_ROOT environment variable. It is nice to have access to this, as often the p4 sync location can be different from the current workspace.
I have a merge request incoming for this.
EDIT: Pull request created at https://github.com/jenkinsci/p4-plugin/pull/77
- links to
Adding P4_Root to environment variable. Submitted to personal forked repo at https://github.com/kroutley/p4-plugin-1/commit/a1b0aa6742b1529764f015257d58f136004d610b