[bifrost] Force Bifrost to use the correct u-c

Following up on [1], Bifrost also needs to use correct u-c and,
surprise-surprise, this is set via a third variable, this time
environment one for the bash script. Oh well. Let's do it.
This has the added benefit of not requiring additional
download of u-c as it reuses the already existing one.

[1] https://review.opendev.org/c/openstack/kolla/+/842276

Change-Id: I2cc79c476d089977b57006dd4ce2e9a8c6206aa1
(cherry picked from commit 633c89b)

Author: yoctozepto

docker/bifrost/bifrost-base/Dockerfile.j2

@@ -40,7 +40,7 @@ RUN apt-get update && \
 {%- else %}
 RUN echo " " && \
 {%- endif %}
-    bash -c './scripts/env-setup.sh && \
+    bash -c 'TOX_CONSTRAINTS_FILE=/requirements/upper-constraints.txt ./scripts/env-setup.sh && \
     ansible-playbook -vvvv -i /bifrost/playbooks/inventory/target /bifrost/playbooks/install.yaml \
     -e @/tmp/build_arg.yml && \
 {%- if base_package_type == 'deb' %}