The conclusion you indicate is obtained as the main result in the following paper, but with an apparently stronger hypothesis: (EDIT: Not stronger at all, actually - just realized you're assuming the map is linear.)
Determinant preserving maps on matrix algebras
Gregor Dolinar and Peter Semrl
Linear Algebra and its Applications Volume 348, Issues 1-3, 15 June 2002, Pages 189-192, DOI: 10.1016/S0024-3795(01)00578-X
Let $M_n$ be the algebra of all $n\times n$ complex matrices. If $\phi:M_n→M_n$ is a surjective mapping satisfying $\det(A+\lambda B)=\det(\phi(A)+\lambda\phi(B))$ then either $\phi$ is of the form $\phi(A)=MAN$ or $\phi$ is of the form $\phi(A)=MA^TN$ where $M,N$ are nonsingular matrices with $\det(MN)=1$.