cmd/go: set $GOROOT during 'go tool' invocations

cmd/dist now requires $GOROOT to be set explicitly.
Set it when invoking via 'go tool dist' so that users are unaffected.

Also, change go tool -n to drop trailing space in output
for 'go tool -n <anything>'.

Change-Id: I9b2c020e0a2f3fa7c9c339fadcc22cc5b6cb7cac
Reviewed-on: https://go-review.googlesource.com/3011


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>