cmd/dist: skip building std for the host when cross-compiling

cmd/dist: skip building std for the host when cross-compiling

The purpose of building the host toolchain is so that we can use it to
build and test the target configuration.

The host configuration should already be tested separately (with its
own builder), so we do not need to build the parts of that
configuration that are not relevant to the task of building and
testing the target configuration.

Updates #47257.

Change-Id: I814778d2d65b1f2887c9419232b5bfd4068f58af
Reviewed-on: https://go-review.googlesource.com/c/go/+/461676
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>