cmd/release: remove Build
cmd/release: remove Build

Change all the methods to free functions. Use workflow.TaskContext to
carry the logger around.

Lots of flags still floating around. Those'll come later.

For golang/go#51797.

Change-Id: I07f1447bd509e2cf6b5e1e4d0f94ebb9f0041d3b
Reviewed-on: https://go-review.googlesource.com/c/build/+/395537
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Reviewed-by: Alex Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

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