Apply go fix modernizations

cmd/qrunproxy/timeline.go

@@ -49,12 +49,7 @@ func (t *TimelineTrack) cellTypeAt(index int, types ...CellType) bool {
 	if index < 0 || index >= len(t.Cells) {
 		return false
 	}
-	for _, typ := range types {
-		if t.Cells[index].Type == typ {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(types, t.Cells[index].Type)
 }
 
 func (c *TimelineCell) String() string {
@@ -98,14 +93,15 @@ func (g exclusiveGroup) satisfied(tracks []*TimelineTrack) bool {
 }
 
 func (g exclusiveGroup) String() string {
-	s := "exclusive("
+	var s strings.Builder
+	s.WriteString("exclusive(")
 	for i, m := range g.members {
 		if i > 0 {
-			s += ", "
+			s.WriteString(", ")
 		}
-		s += m.String()
+		s.WriteString(m.String())
 	}
-	return s + ")"
+	return s.String() + ")"
 }
 
 func (tl *Timeline) debugf(format string, args ...any) {