cmd/gopherbot: add the list of tasks in the help message
cmd/gopherbot: add the list of tasks in the help message

Change-Id: I1c2456ad1854712492e5cb625e7652e6ed030229
Reviewed-on: https://go-review.googlesource.com/c/build/+/401694
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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