ci: Fix publishing to pages from default branch

Commit 728c9142 "Use CI_DEFAULT_BRANCH as the default branch" actually
broke the publishing via GitLab pages from the default branch since
the `only: [ ${CI_DEFAULT_BRANCH} ]` condition was not really expanding
the variable and it was considering `${CI_DEFAULT_BRANCH}` as a literal
branch name.

This means that the `test-pdf` job got always instantiated while `pages`
never was, regardless of the branch they where running on.

By switching to `rules:` we can properly implement automatic default
branch detection and fix the regression.

Signed-off-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>