Enable rustfmt on AppVeyor again

[phansch]

#4617 disabled running rustfmt on AppVeyor because of some cargo issues.

Specifically the error is: error: no such subcommand: '+nightly'.

Link to full AppVeyor log

[flip1995]

#4624 found a solution with rustfmt still enabled on appveyor: https://ci.appveyor.com/project/rust-lang-libs/rust-clippy/builds/27872685#L893