CI: Fix the git commit range we pass on to gitlint

The check-compliance.py script uses gitlint to verify the commit message
and takes a range of git commits to validate via the env var
${COMMIT_RANGE}.  Previously we used ${SHIPPABLE_COMMIT_RANGE} however
for branches that doesn't get us what we want.  So we move to using
origin/${PULL_REQUEST_BASE_BRANCH}..${COMMIT}.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>