cmd/tip: also fetch x/net repository before building cmd/godoc
cmd/tip: also fetch x/net repository before building cmd/godoc

x/tools/cmd/godoc uses at least one Go package from x/net as of
CL 157197.

Don't add it to signature because we don't want the signature to
change whenever new commits to x/net are pushed, causing tip.golang.org
to be redeployed. This is because x/net is not considered a critical
component of the website at this time, and that's not expected to
change soon.

When the website begins using modules, it will specify the x/net
version precisely and this decision will no longer matter.

Fixes golang/go#29874

Change-Id: I1fa76bb81f8d2ffc2314375e2dfe4898c3af58de
Reviewed-on: https://go-review.googlesource.com/c/158937
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build