CI: execute run_fetch_branches_tool only when testing a PR
CI: execute run_fetch_branches_tool only when testing a PR

run_fetch_branches_tool is designed to run successfully
only for testing PRs. Adding a validation to run only when
env variable "PULL_REQUEST_NUMBER" exists.

Fixes #156.

Signed-off-by: Salvador Fuentes <1643e95c8f87731315a493a18f5448396bacc6be@intel.com>

GabyCT/tests,sboeuf/tests,chavafg/tests,bwarden/tests,clearcontainers/tests,clearcontainers/tests,devimc/tests,chavafg/tests,MarioCarrilloA/tests,MarioCarrilloA/tests,devimc/tests,bwarden/tests,sboeuf/tests,GabyCT/tests