Scheduled build: Add env var with schedule id

[jmendiara]

It would be nice to have the Schedule uuid available as an env var when a build is triggered by a schedule.

example:

BUILDKITE_SCHEDULED_FROM_ID=327d9a14-754e-4a98-86e3-f4bb1a53012d

This way we can easily react and update schedule configuration inside the builds (using graphql api). Examples:

• Disable schedule if the triggered pipeline fails
• Enable it after user review
• Delay schedules by a timeframe