build: add alternate output format for bootstrap.bash, as used by builders

build: add alternate output format for bootstrap.bash, as used by builders

I've been doing these tweaks by hand. I was going to write a tool in
Go for it, but it's not much additional shell here.

Fixes #22912
Updates #9797 (already closed)

Change-Id: Ia15bd9b6876e6f6a76aa9ca86b10f113095e96a3
Reviewed-on: https://go-review.googlesource.com/80895
Reviewed-by: Ian Lance Taylor <iant@golang.org>