For \( a \in \mathbb{R} \), consider the system of linear equations
\[
\begin{cases}
a x + a y = a + 2,
x + a y + (a - 1)z = a - 4,
a x + a y + (a - 2)z = -8,
\end{cases}
\]
in the unknowns \(x, y, z\). Then, which of the following statements is TRUE?