cmd/genbuilderkey: add usage information
cmd/genbuilderkey: add usage information

This change adds usage information to the genbuilderkey command.
This defines what the requirements and arguments are for executing
the command.

Change-Id: Ie6c465f682c4f78a34a30db09a0e710dd071b4b8
Reviewed-on: https://go-review.googlesource.com/c/build/+/232217
Run-TryBot: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

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