Fixes TestGoGetWorksWithVanityWildcards,
but that test uses the network and is not run
on the builders.

For #11806.

Change-Id: I35c6677deaf84e2fa9bdb98b62d80d388b5248ae
Reviewed-on: https://go-review.googlesource.com/12557Reviewed-by: Ian Lance Taylor <iant@golang.org>