dev/sg: add 'sg ci open'

Created by: bobheadxi

sg ci open opens the Buildkite page for the Sourcegraph org, or the specified arbitrary pipeline.

Part of https://github.com/sourcegraph/sourcegraph/issues/31005 . Small idea rasied https://docs.google.com/document/d/1NS_C3te-59P149LD_rrL1zcflNqqZOoSHVJfv8oPcqY/edit

Test plan

sg update -local
sg ci open
sg ci open deploy-sourcegraph