Pass cancel grace period to bootstrap

[DrJosh9000]

Description

The buildkite-agent bootstrap forked from buildkite-agent start needs both cancel-grace-period and signal-grace-period.

Context

In #2899 the cancel grace period was added as a flag and env var read by the bootstrap subcommand, but I forgot to set it from the job runner.

Changes

• Pass the value through an env var just like BUILDKITE_SIGNAL_GRACE_PERIOD_SECONDS
• Also a small cleanup across the whole agent codebase to replace fmt.Sprintf("%d", ...) with strconv.Itoa(...) (does the same thing, but it doesn't need the fmt package to parse a format string).

Note: the two env vars are different due to historical reasons. See https://github.com/buildkite/agent/blob/main/clicommand/cancel_signal.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 fmt ./...)
• Manually checked that it works

[wolfeidau]

👍🏻 much better using strconv.Itoa 🚀