bk local run - support if conditions - currently ignoring them

[drnic]

https://buildkite.com/docs/pipelines/conditionals

steps:
  - command: "echo --- All good. Nice one."
    label: ":git:"
    if: build.branch == 'master'
  - command: "echo --- ERROR this shouldn't run; exit 1"
    label: ":git:"
    if: build.branch != 'master'

When I run this pipeline.yml it runs both steps; but it should only run one or the other.

@keithpitt is this something that should work, or one of those "its annoying to implement everything in ruby + golang" (my words not yours) gaps ;)