Use commit constant in conditional

Co-authored-by: Olivier Halligon <olivier.halligon@automattic.com>

Author: iangmaia

lib/fastlane/plugin/wpmreleasetoolkit/actions/common/buildkite_upload_pipeline_action.rb

@@ -11,7 +11,7 @@ def self.run(params)
         commit = params[:commit]
 
         UI.user_error!("Pipeline file not found: #{pipeline_file}") unless File.exist?(pipeline_file)
-        UI.user_error!('You should not provide both `branch` and `commit`') if !branch.nil? && commit != 'HEAD'
+        UI.user_error!('You should not provide both `branch` and `commit`') if !branch.nil? && commit != DEFAULT_COMMIT
         UI.user_error!('This action can only be called from a Buildkite CI build') unless ENV['BUILDKITE'] == 'true'
 
         UI.message "Uploading pipeline on #{pipeline_file}, #{"branch #{branch}, " if branch}commit #{commit}"