stage0 command

The stage0 command looks up the buildlet's URL from its environment (GCE metadata service, EC2, etc), downloads it, and runs it. If not on GCE, such as when in a Linux Docker container being developed and tested locally, the stage0 instead looks for the META_BUILDLET_BINARY_URL environment to have a URL to the buildlet binary.

The stage0 binary is typically baked into the VM or container images or manually copied to dedicated once and is typically never auto-updated. Changes to this binary should be rare, as it's difficult and slow to roll out. Any per-host-type logic to do at start-up should be done in x/build/cmd/buildlet instead, which is re-downloaded once per build, and rolls out easily.