cmd/buildlet/stage0: use httpdl instead of curl for getting bootstrap tarball
cmd/buildlet/stage0: use httpdl instead of curl for getting bootstrap tarball

Fixes golang/go#16246

Change-Id: I3817f9bc7948113443e51174be21580ac43b4fbf
Reviewed-on: https://go-review.googlesource.com/c/build/+/179421
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build