cmd/buildstats: fix breaking API change in dependency

CL https://code-review.googlesource.com/#/c/9567/ unexported
bigquery.ValueList, which broke the code in this project. Switch the
buildstats code per the suggestion in the linked commit.

Fixes golang/go#18764.

Change-Id: I55b748f65732d5627d966623cd3b0e73223b16be
Reviewed-on: https://go-review.googlesource.com/35572
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed