fix(nix): use nix gcc and provide libiconv (#916)

* fix(nix): use nix gcc and provide libiconv

Fixes iconv problems on darwin when using nix's gcc by removing
an impurity from build.

Adding this is apparently a no-op for linux, because libiconv is included by default in libc for linux.
This allows us to omit conditionals.

Before:
```sh
❯ otool -L libblink_cmp_fuzzy.dylib
libblink_cmp_fuzzy.dylib:
        /Users/konrad/Code/github.com/konradmalik/blink.cmp/target/release/deps/libblink_cmp_fuzzy.dylib (compatibility version 0.0.0, current version 0.0.0)
        /usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1351.0.0)
```

After:
```sh
❯ otool -L ./target/release/libblink_cmp_fuzzy.dylib
./target/release/libblink_cmp_fuzzy.dylib:
        /Users/konrad/Code/github.com/konradmalik/blink.cmp/target/release/deps/libblink_cmp_fuzzy.dylib (compatibility version 0.0.0, current version 0.0.0)
        /nix/store/v7ldx1ra3wrjaasap8bfradapkqi2w1r-libiconv-107/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.0.0)
```

* ci(nix): check, build, run, even on prs

Also adds nix-comminity cache to avoid builds from fenix.

.github/workflows/nix.yaml

@@ -0,0 +1,38 @@
+name: Test Nix
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    name: Test Nix Build on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@main
+        with:
+          extra-conf: |
+            accept-flake-config = true
+
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+
+      - name: Check the flake
+        run: nix flake check
+
+      - name: Build devshell
+        run: nix develop --command "rustc"
+
+      - name: Build the library
+        run: nix build .#blink-fuzzy-lib
+
+      - name: Build the plugin in nix
+        run: nix build .#blink-cmp
+
+      - name: Build the library (outside nix)
+        run: nix run .#build-plugin

flake.nix

@@ -83,13 +83,9 @@
           program = let
             buildScript = pkgs.writeShellApplication {
               name = "build-plugin";
-              runtimeInputs = with pkgs;
-                [
-                  fenix.minimal.toolchain
-                ]
-                # use the native gcc on macos, see #652
-                ++ lib.optionals (!pkgs.stdenv.isDarwin) [ gcc ];
+              runtimeInputs = with pkgs; [ fenix.minimal.toolchain gcc ];
               text = ''
+                export LIBRARY_PATH="${lib.makeLibraryPath [ pkgs.libiconv ]}";
                 cargo build --release
               '';
             };
@@ -110,4 +106,11 @@
         formatter = pkgs.nixfmt-classic;
       };
     };
+
+  nixConfig = {
+    extra-substituters = [ "https://nix-community.cachix.org" ];
+    extra-trusted-public-keys = [
+      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs"
+    ];
+  };
 }