cmd/go: propagate context into Action.Func calls

Action.Func is now a func(*Builder, context.Context, *Action), so that
contexts can be propagated into the action funcs. While context
is traditionally the first parameter of a function, it's the second
parameter of Action.Func's type to continue to allow for methods
on Builder to be used as functions taking a *Builder as the first
parameter. context.Context is instead the first parameter on
those functions.

Change-Id: I5f058d6a99a1e96fe2025f2e8ce30a033d12e935
Reviewed-on: https://go-review.googlesource.com/c/go/+/248321


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>