cmd/go: disable sumdb less often for toolchain downloads

cmd/go: disable sumdb less often for toolchain downloads

There is a chicken and egg problem with always requiring
the checksum database for toolchain module downloads, since the
checksum database populates its entry by doing its own module
download.

Don't require the checksum database for GOPROXY=file:/// (for local testing)
and when running on the Go module mirror.

For #60847.

Change-Id: I5d67d585169ae0fa73109df233baae8ba5fe5dd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/503978
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>