Using that modulo $\;2\;$ we have $\;(a+b)^2=a^2+b^2\;$ :
$$x^7-x=x(x^6-1)=x(x^3-1)^2=x(x-1)^2(x^2+x+1)^2$$
Of course, $\;-1=1\;$ so you can write $\;x-1\;$ or $\;x+1\;$ . It's the same.
Using that modulo $\;2\;$ we have $\;(a+b)^2=a^2+b^2\;$ :
$$x^7-x=x(x^6-1)=x(x^3-1)^2=x(x-1)^2(x^2+x+1)^2$$
Of course, $\;-1=1\;$ so you can write $\;x-1\;$ or $\;x+1\;$ . It's the same.