fix regression when using Run twice on the first ctx

We don't want to always set c.first, as that can change the field from
true to false.

context.go

@@ -114,13 +114,13 @@ func Run(ctx context.Context, actions ...Action) error {
 	if c == nil || c.Allocator == nil {
 		return ErrInvalidContext
 	}
-	c.first = c.Browser == nil
-	if c.first {
+	if c.Browser == nil {
 		browser, err := c.Allocator.Allocate(ctx)
 		if err != nil {
 			return err
 		}
 		c.Browser = browser
+		c.first = true
 	}
 	if c.Target == nil {
 		if err := c.newSession(ctx); err != nil {

context_test.go

@@ -41,4 +41,10 @@ func TestTargets(t *testing.T) {
 	// Cancelling the second context should close the second tab alone.
 	cancel2()
 	wantTargets(ctx1, 1)
+
+	// We used to have a bug where Run would reset the first context as if
+	// it weren't the first, breaking its cancellation.
+	if err := Run(ctx1); err != nil {
+		t.Fatal(err)
+	}
 }