serde_derive compilation fails on windows gnu with rust nightly

See example build at:

https://ci.appveyor.com/project/sagiegurari/cargo-make/build/1.0.600/job/tt03ws1pdppwq6u2#L176

I see that you do not run builds on gnu in appveyor so you will not see it.
So I suggest apart of a fix to also add it to the appveyor matrix.