update post-merge slack failure message
update post-merge slack failure message

this encourages the behavior of alerting the owning team if spec
flakiness is identified and adds a link to the spec tracker
dashboard for easier investigation

Change-Id: I68ca319866a0aa35bb306b4cb5a2791135669659
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/272131
Tested-by: Andrea Cirulli <b4e55ef5ee4bae8ad31f7c81009bfc0af1714a1f@instructure.com>
Reviewed-by: Andrea Cirulli <b4e55ef5ee4bae8ad31f7c81009bfc0af1714a1f@instructure.com>
QA-Review: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
Product-Review: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms