Rotate object to a certain rotation if key press

How do I make a object rotate to just X=20 if Lshift is pressed
and then go back to X=90 if Lshift is released?

if(Input.GetKeyDown(KeyCode.LeftShift) {
transform.rotation = Quaternion.Euler(20,0,0);
transform.rotation = Quaternion.Euler(90,0,0);

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