Backport https://github.com/go-gitea/gitea/pull/38984
Workflows using YAML anchors are rejected as invalid, because a workflow
is split into one document per job and an alias whose anchor lands in
another job's document no longer resolves. Node walkers such as `on:`
parsing have no alias case either.
Aliases are now expanded once, right after the workflow is parsed and
before anything reads or splits it, bounded like GitHub's parser so
nested aliases cannot expand without limit. Merge keys stay unsupported,
as they are upstream.
Fixes: https://github.com/go-gitea/gitea/issues/38983
