swiper.el (swiper-font-lock-exclude): Add nix-mode

[eqyiel]

The first time swiper is invoked upon switching to a nix-mode buffer swiper--candidates will say End of buffer during the save-excursion. Invoking it a second time swiper--candidates will return normally.

If this is what swiper-font-lock-exclude is used to prevent, please consider this PR!

[eqyiel]

I may have jumped the gun a bit; this doesn't happen for all nix-mode buffers. One file that I see this behaviour for can be found here though: https://github.com/eqyiel/nixos-config/blob/master/config/localghost/configuration.nix

[abo-abo]

Thanks.