cmd/tip: use website repo on disk for installing cmd/golangorg
cmd/tip: use website repo on disk for installing cmd/golangorg

The 'go install golang.org/x/website/cmd/golangorg' command is being
configured with GO111MODULE=on and GOPROXY=https://proxy.golang.org
environment variables, but its directory was not set. As a result,
the main module was an empty module (GOMOD=/dev/null) instead of
the module in the website repository that was just checked out.

Set the working directory of the go install command to websiteDir,
so that the intended website module is used during the build.

Fixes golang/go#34545

Change-Id: If51030494ddb90a63b9d198209a04c5fb328784e
Reviewed-on: https://go-review.googlesource.com/c/build/+/197397
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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