L2SS-259: Pin PBR_VERSION in gitlab-ci

This is actually unused as we have our own method to determine
the version for packaging.

.gitlab-ci.yml

 image: artefact.skatelescope.org/ska-tango-images/tango-itango:9.3.3.7
 variables:
   PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
+  PBR_VERSION: "0.1"
 cache:
   paths:
     - .cache/pip