Add the environment's PYTHONPATH to sys.path when running dev pants.

This is helpful when running a graphical debugger: you need to ensure that
the debugger's code (pydevd in my case) is on your path so you can import
it at the tracepoint.

Reviewed at https://rbcommons.com/s/twitter/r/1237/

pants

@@ -77,7 +77,7 @@ function run_pants_bare() {
 
 function exec_pants_bare() {
   activate_pants_venv
-  PYTHONPATH="${PANTS_SRCPATH}" exec python ${PANTS_EXE} "$@"
+  PYTHONPATH="${PANTS_SRCPATH}:${PYTHONPATH}" exec python ${PANTS_EXE} "$@"
 }
 
 if [ ! -z "${PANTS_DEV}" ]; then