Backport(v6): ci: fix empty base_ref error (#831) (#937)

Backport #831
Backport #830

## #831
Try to use $Env:GITHUB_REF_NAME which contains branch name

github.base_ref is empty when executing via workflow_dispatch.

## #830
When kicked from LTS branch, base branch should be set explicitly,
otherwise PR will be created against main branch.

---------

Signed-off-by: Kentaro Hayashi <[email protected]>
Signed-off-by: Shizuo Fujita <[email protected]>
Co-authored-by: Kentaro Hayashi <[email protected]>

Author: Watson1978

.github/workflows/maintenance-bundle-update.yml

@@ -34,4 +34,4 @@ jobs:
           git add fluent-package/Gemfile.lock
           git commit --message "Update Gemfile.lock" --signoff
           git push origin $branch
-          gh pr create --title "Update Gemfile.lock" --body "Update Gemfile.lock by maintenance workflow"
+          gh pr create --base $Env:GITHUB_REF_NAME --title "Update Gemfile.lock" --body "Update Gemfile.lock by maintenance workflow"