Allow supplementing $PATH for RHEL/Debian

To avoid supplementing `$PATH` by hand-modifying a Jenkins server in the "Manage Jenkins" page or using absolute paths in job configs, this will stick a `PATH=$PATH:<your-extra-paths>` into the shell configuration file used during Jenkins startup.

If there is a better way of doing this, I am all ears. After reading the documentation and `grep`ing the code, I didn't find what I needed.