cmd/go: omit checksums for go.mod files needed for go version lines more often in...

cmd/go: omit checksums for go.mod files needed for go version lines more often in pre-1.21 modules

This updates the logic from CL 489075 to avoid trying to save extra
sums if they aren't already expected to be present
and cfg.BuildMod != "mod" (as in the case of "go list -m -u all" with
a go.mod file that specifies go < 1.21).

Fixes #60667.
Updates #56222.

Change-Id: Ied6ed3e80a62f9cd9a328b43a415a42d14481056
Reviewed-on: https://go-review.googlesource.com/c/go/+/502015
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>