Add support for GIT LFS checkout

[lizrabuya]

Description

Add configuration and environment variables when the backend sets BUILDKITE_GIT_LFS_ENABLED=true in the job environment. When BUILDKITE_GIT_LFS_ENABLED=true is set, the agent will check if the git-lfs binary is installed and proceed to perform a git lfs fetch + git lfs checkout.

Changes

Configuration & CLI (clicommand/bootstrap.go, internal/job/config.go)

• Added a new GitLFSEnabled bool field to ExecutorConfig, backed by env var BUILDKITE_GIT_LFS_ENABLED
• Exposed it as a --git-lfs-enabled CLI flag on the bootstrap command with usage description
• Wired the flag through from BootstrapConfig into ExecutorConfig

Checkout behaviour (internal/job/checkout.go)

• Added a fail-fast binary check: if GitLFSEnabled is true and git-lfs is not on PATH, the checkout fails immediately with a descriptive error before any fetch/checkout work
• Added git lfs install --local after clone/clean but before fetch, so the LFS filter is registered in the local repo config
• Added a call to gitLFSFetchCheckout after the main git checkout (and after submodule updates) to explicitly pull down and materialise LFS objects

LFS fetch helper (internal/job/git.go)

• Added gitLFSFetchCheckout, which runs git lfs fetch then git lfs checkout, wrapping each failure with a distinct error prefix ("git lfs fetch: ..." / "git lfs checkout: ...") so the
  failing step is identifiable from logs alone

Smudge bypass (internal/job/executor.go)

• Set GIT_LFS_SKIP_SMUDGE=1 in setUp so LFS objects are never downloaded implicitly during git checkout; the explicit gitLFSFetchCheckout call handles download instead

Tests (internal/job/checkout_test.go)

• Added TestDefaultCheckoutPhase_GitLFS covering four scenarios: LFS disabled, LFS enabled with binary present, LFS enabled with binary missing (PATH restricted via t.Setenv), and LFS
  enabled with a failing git lfs command (fake git-lfs script that exits 1)

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

The main function changes were implemented by me based on the recommendation from the research done for this function. I used Claude to review the code and to assist with writing the tests.