all: refer to build.golang.org by HTTPS URLs only

Most of these are documentation fixes.

Change-Id: If100cc2ef0ee0b8b47a1e96e4c51a2be8caf5842
Reviewed-on: https://go-review.googlesource.com/22366
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
6 files changed