Add PORTAGE_PYM_PATH variable when calling portageq.

When portageq was called in bash v5.1, portageq passed despite the
PORTAGE_PYM_PATH environment variable not being set. This was because
v5.1 returned an empty string for unset variables, causing the "cd" call
to default to HOME. However, in bash v5.3, unset variables return null,
causing all chromeos-base packages to fail. This change explicitly sets
this variable.

BUG=b/454135047
TEST=presubmit
RELEASE_NOTE=None

Change-Id: I3a306d0ad4598c20f085d3a9902e63bb6c24089a
Reviewed-on: https://cos-review.googlesource.com/c/third_party/overlays/eclass-overlay/+/123761
Reviewed-by: He Gao <hegao@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>