delete build.nix and the nix tests

From what I have seen from other projects, they do not maintain a
`build.nix` file themselves, nor do they test specifically with `nix`. I
believe this should be under the purview of the `nix` maintainers.

I do not know enough about `nix`'s packaging system to reliably maintain
this. Furthermore, it has been poluting our CI for a while, since it
takes much longer to run than the other tests.

If there is ever any problem building in `nix`, we can fix that as it
comes up. Alternatively, if someone else wishes to create an automatic
CI test for `nix`, go ahead. But I am simply turning it off again the
moment it stops working and I can't fix it within an hour.

Author: LGFae

.github/workflows/ci.yml

@@ -14,5 +14,3 @@ jobs:
     uses: ./.github/workflows/dependencies.yml
   test:
     uses: ./.github/workflows/test.yml
-  nix-tests:
-    uses: ./.github/workflows/nix-tests.yml

.github/workflows/nix-tests.yml

@@ -41,6 +41,8 @@
         pkg-config,
         lz4,
         libxkbcommon,
+        wayland,
+        wayland-protocols,
         swww,
       }:
         mkShell {