cmd/go: set dependency go versions for go install --mod=readonly mod@vers

cmd/go: set dependency go versions for go install --mod=readonly mod@vers

When running go install --mod=readonly module@version. modfetch.GoSumFile
was not set, so the checksumOk check that's done when checking whether
we need to set the GoVersion from the go mod file was failing. Bypass
the checksumOk check when there's no GoSumFile.

For #54908

Change-Id: I56cf9d36a505b1223e6bf82a7d455746e2f09849
Reviewed-on: https://go-review.googlesource.com/c/go/+/439855
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>