cmd/go: disallow go.sum updates in -mod=readonly

cmd/go: disallow go.sum updates in -mod=readonly

When running go build with the flag -mod=readonly, it fails the build if
go.sum files requires updating. This ensures that CI/CD systems get a
complete go.sum file so that they'd never hit a notary,
assuming the CI/CD system passes the above flag.
I am not familiar with the entire codebase but I assume goSum.dirty
will always be true if go.sum has any missing lines.

Fixes #30667

Change-Id: I767d3b594055d8c10048f4c68e6687c94bb0545c
Reviewed-on: https://go-review.googlesource.com/c/go/+/166237
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>