`TANGLED_REPO_DEFAULT_BRANCH` not populated in pipeline runs as documented

Per https://docs.tangled.org/spindles#environment there should be a TANGLED_REPO_DEFAULT_BRANCH env var available in spindle workflow runs.

When I add this to a workflow:

echo "DEBUG: TANGLED_REPO_DEFAULT_BRANCH='$TANGLED_REPO_DEFAULT_BRANCH'"

It does not appear to be populated:

DEBUG: TANGLED_REPO_DEFAULT_BRANCH=''