Replace deprecated UPPER_CONSTRAINTS_FILE variable

UPPER_CONSTRAINTS_FILE is deprecated and TOX_CONSTRAINTS_FILE is
the new environment variable name that replaces it [1].

This allows to use upper-constraints file as more
readable way instead of UPPER_CONSTRAINTS_FILE=<lower-constraints file>.

[1] https://zuul-ci.org/docs/zuul-jobs/python-roles.html#rolevar-tox.tox_constraints_file
[2] https://review.opendev.org/#/c/722814/

Change-Id: I0d50f2938ad2dbee418e630f6fdb3c0da5b36e24