Support checkout skipping in agent

[mcncl]

Description

The kubernetes stack allows checkout: skip: true to be set in the step config, but using non-k8s doesn't support this without the use of a plugin. By adding this flag/env var reading we allow for a future where skip_checkout: true can be set in the step (BUILDKITE_SKIP_CHECKOUT)

This also allows agents to run with --skip-checkout, which means folks can ensure that they can have a fleet of agents in a certain queue which won't checkout.

Context

A lot of folks use third party plugins to achieve this, and the k8s supports it as a first class citizen.

Changes

• adds SkipCheckout as an agent configuration option
• sets the job checkout to determine if skipping checkout has been set (this is for a future skip_checkout config option
• enabled users to set a flag, --skip-checkout in start and bootstrap
• adds a test to checkout_test.go and checkout_integration_test.go

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

All me babyyyyyyy.

[mcncl]

Keen on knowing, too, if we already have plans to support the existing pattern;

steps:
  - label: "Alpaca"
    checkout:
      skip: true

If so I'll happily adjust this PR so it doesn't look for a future config state.

[zhming0]

Looks good to me! 💚