gitlab-ci.yml: fix rules:if for build-docs job

There are two separate issues here:

- CI_BRANCH_NAME doesn't exist, we want CI_COMMIT_BRANCH instead.
- The rules engine doesn't handle quotes in the normal sense.
Quotes are only allowed on literals, not variable references.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>

.ostc-ci/gitlab-ci.yml

@@ -60,7 +60,7 @@ build-docs:
         # Run this job in case the pipeline changes.
         - .ostc-ci/*.yml
     # Or when things land.
-    - if: '$CI_BRANCH_NAME == "$CI_DEFAULT_BRANCH"'
+    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
       changes:
         # React to changes to the docs directory.
         - docs/**/*