bootstrap/config.go(Config): Signpost struct-tag magic.
bootstrap/config.go(Config): Signpost struct-tag magic.

I ran into an issue where even though i'd added the `env:""` struct tag, the config wasn't getting
mapped from the environment when testing locally with `buildkite-agent bootstrap`.  My understanding
might be flawed, but even though i was setting `BUILDKITE_PLUGINS_FORCE_PULL` and _not_ using any
CLI flag for it, the env variable config only got picked up once i added the `cli:..` annotation and
config option to clicommand/bootstrap.go(BootstrapConfig).

Let me know if i've misunderstood.

buildkite/agent,buildkite/agent,buildkite/agent