cmd/internal/doc: fix GOROOT skew and path joining bugs
Use the goCmd() function to get the go command to invoke, so that when
GOROOT is set, the go command that's invoked uses the same GOROOT.
Otherwise there will be skew between the go command and the tools and
runtime. Also use the environment when determining GOPROXY and
GOMODCACHE, and use url.Join so the slashes in 'http://' aren't
collapsed into one.
Change-Id: Ie36ca2fffdb015a7f5f9bd7f514850e41fad2c1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/685319 Reviewed-by: Michael Matloob <matloob@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>