test: use flux env in maybe-installtest

problem: while the sched PYTHONPATH is set reliably by a mix of cmake
and maybe-installtest, the flux PYTHONPATH is not

solution: wrap the test in an invocation of `flux env` to set the
PYTHONPATH appropriately allowing the python shebang in python tests to
succeed

fixes #1172

t/scripts/maybe-installtest

@@ -4,4 +4,4 @@ if test -n "$FLUX_SCHED_TEST_INSTALLED" -o -n "$FLUX_TEST_INSTALLED_PATH"; then
   export FLUX_MODULE_PATH_PREPEND="${FLUX_SCHED_MOD_DIR}"
   export PYTHONPATH="${FLUX_SCHED_PYTHON_SITELIB}${PYTHONPATH:+:${PYTHONPATH}}"
 fi
-exec $@
+exec flux env $@