Context.BuildTags is not set when you read go/build.Default.BuildTags.
It's only used by (*BuildTags).Import, etc.

Fixes: #27320

Change-Id: I97e5f1923c410b48f70be8c15938a7e04a178e3f
Reviewed-on: https://go-review.googlesource.com/c/131975Reviewed-by: Ian Lance Taylor <iant@golang.org>