crypto/internal/fipstest: fix TestACVP not to write to module cache

crypto/internal/fipstest: fix TestACVP not to write to module cache

Not sure how this ever worked; the module cache is read-only.

Change-Id: I22e4945fa0bac101995ab9101d22dcfdcee40776
Reviewed-on: https://go-review.googlesource.com/c/go/+/629976
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>