cmd/go/internal/work: avoid panic for a repeated //go:debug setting

cmd/go/internal/work: avoid panic for a repeated //go:debug setting

The creation of a bytes.Buffer in one code path is missing causing a nil
pointer dereference.

Changed (as rec. by Bryan Mills) to use fmt.Appendf() on []byte instead of
fmt.Fprintf on *bytes.Buffer - simpler and avoids duplicated code (but
requires Go 1.19 or later).

Added test to verify the change (as rec. by Michael Matloob) at
src\cmd\go\testdata\script\build_repeated_godebug_issue62346.txt

Fixes #62346

Change-Id: Ic3267d878a6f7ebedb1cde64e6206de404176b10
Reviewed-on: https://go-review.googlesource.com/c/go/+/523836
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Michael Matloob <matloob@golang.org>