cmd/go: add missing newline to "go" line in cache key

cmd/go: add missing newline to "go" line in cache key

This was missed in CL 223139. It doesn't seem to affect correctness,
but might be confusing if we need to debug the cache key.

Updates #37804

Change-Id: I979efa68381cf79a7e246581510c90a724be6cd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/223144
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>