MLE of a Weibull Scale with Known Shape

Suppose execution delays are modeled as Weibull with known shape $k=2$ and unknown scale $\lambda$, with density $$f(x)=\frac{2x}{\lambda^2}e^{-(x/\lambda)^2}, \qquad x>0.$$ If $n=10$ observations satisfy $$\sum_{i=1}^{10} X_i^2 = 90,$$ find the MLE of $\lambda$.