[release-branch.go1.26] cmd/go: revert update default go directive in mod or work...

[release-branch.go1.26] cmd/go: revert update default go directive in mod or work init

This restores the previous behavior of setting go directive to the toolchain's version as per #77653.

For #77653
Fixes #77860

Change-Id: Ie9d2de025a75f39fd8d6d01776d0cf4e5da954f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/749948
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Austin Clements <austin@google.com>