cmd/go: adjust conditions in which toolchain lines are written

cmd/go: adjust conditions in which toolchain lines are written

'go mod tidy -go=1.20' should tidy as Go 1.20 did, without writing a
toolchain line implicitly. (We don't need it to stabilize toolchain
version switching anyway: because Go 1.20 predates toolchain
switching, any toolchain that supports switching toolchains also
supports Go 1.20 modules directly.)

For #57001.

Change-Id: I415abac75d8d6de9f8ed470aab0d1ed4c225b08d
Reviewed-on: https://go-review.googlesource.com/c/go/+/499987
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>