database/sql: allocate once when assigning a time.Time to a byte slice

Change convertAssignRows to use time.AppendFormat and a pre-allocated
buffer when assigning a time.Time to a byte slice. Previously, the
result of time.Format was converted to a byte slice which required
two allocations.

Change-Id: I19db5e4d295e882070f947eca318a4e33520cda1
Reviewed-on: https://go-review.googlesource.com/c/go/+/622597
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/database/sql/convert.go

@@ -290,7 +290,7 @@ func convertAssignRows(dest, src any, rows *Rows) error {
 			if d == nil {
 				return errNilPtr
 			}
-			*d = []byte(s.Format(time.RFC3339Nano))
+			*d = s.AppendFormat(make([]byte, 0, len(time.RFC3339Nano)), time.RFC3339Nano)
 			return nil
 		case *RawBytes:
 			if d == nil {