Build with ghc-8.2.2 using Nix

[PierreR]

This is not ready to be merged.

I would like to discuss this PR to get feedback and help.

This PR allows me to install Taffybar compiled with ghc-8.2.2. This is necessary because of #358.

You can try it out in nixos (and I guess also with nix and other linux distro) by using this command:

nix-env -i -f https://github.com/pierrer/taffybar/tarball/20584dafc952fbe5ee36136e0492500749ea7faa

It is going to take quite a while because a lot of the dependencies are not in the nixos default binary cache.

One issue remains. It works on the stable nixos channel (18.03) but it won't work so well when I use the unstable channel (18.09). In 18.09 it completes the building process without problem but Taffybar will fail to recompile the user configuration. It complains that it can't find the imported taffybar modules.

As a bonus, I would also like to speed up the build/compilation process (by disabling profiling globally, using `justStaticExecutables´, ...). Any idea or suggestion welcomed.

When I am happy about the workflow I will probably push it to cachix

Thanks for your help and feedback.

[colonelpanic8]

Thanks for submitting, this should be helpful to a lot of people. I'll try it out on my machine in the near future.

[PierreR]

@IvanMalison I think it is safe to merge this if you believe it can be useful. I have pushed the nar files into cachix: https://taffybar.cachix.org/. If it works as expected there would be no compilation.
If the default ghc is not ghc8.2.2 the dynamic reloading of the configuration (with dyre) might fail. I still have to figure out that part.

You don't need to install cachix to test it but it provides an easy way to add the cache with cachix use taffybar.

[colonelpanic8]

@PierreR I'm confused. Why does this need to exist at all. Seems like we should just use the existing nixpkgs derivations for taffybar.

[PierreR]

Yep it can be removed now that everything is working correctly in nixpkgs

[PierreR]

I can do a PR to clean this up.