Change Slack notification to use BUILD_JOB_NAME
Change Slack notification to use BUILD_JOB_NAME

[#166809156](https://www.pivotaltracker.com/story/show/166809156)

Co-authored-by: Andrew Crump <bad96bc28586e95c5d7da81c13d926ea5028b22f@pivotal.io>

cloudfoundry/cli,cloudfoundry/cli,cloudfoundry/cli