cmd/go: fix tests for new builder environment

cmd/go: fix tests for new builder environment

Fix two long tests that fail in the builders we're trying out:

- TestQueryImport was failing with:
  open /nonexist-gopath/pkg/sumdb/sum.golang.org/latest: no such file or directory
  which eventually turns out to be because it couldn't create
  /nonexist-gopath because it wasn't running as root. The test already
  uses a temporary GOPATH, but missed overriding a configuration
  variable set at init time.
- test_flags fails if the working directory has /x/ in it, which it now
  happens to.

Change-Id: Ideef0f318157b42987539e3a20f9fba6a3d3bdd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/480255
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>