Use niv out of nixpkgs (#195)

* Use `niv` out of `nixpkgs`
* Update CI tests and add one for Nix formatting

.github/workflows/check.yaml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out codebase
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install nix
         uses: cachix/install-nix-action@v15
@@ -28,11 +28,11 @@ jobs:
       - name: Test
         run: nix-shell --run check-code
 
-  Validate_Format:
+  Validate_Purescript_Format:
     runs-on: ubuntu-latest
     steps:
       - name: Check out codebase
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install nix
         uses: cachix/install-nix-action@v15
@@ -43,13 +43,30 @@ jobs:
         run: nix-shell
 
       - name: Validate Format
-        run: nix-shell --run check-format
+        run: nix-shell --run check-format-purescript
+
+  Validate_Nix_Format:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out codebase
+        uses: actions/checkout@v3
+
+      - name: Install nix
+        uses: cachix/install-nix-action@v15
+        with:
+          nix_path: nixpkgs=channel:nixpkgs-unstable
+
+      - name: Install environment
+        run: nix-shell
+
+      - name: Validate Format
+        run: nix-shell --run check-format-nix
 
   Validate_Pulp:
     runs-on: ubuntu-latest
     steps:
       - name: Check out codebase
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install nix
         uses: cachix/install-nix-action@v15

shell.nix

@@ -2,13 +2,12 @@
   sources ? import ./sources.nix,
   nixpkgs ? sources.nixpkgs,
   easy-purescript-nix ? sources.easy-purescript-nix,
-  niv ? sources.niv,
   alejandra ? sources.alejandra,
 }: let
-  niv-overlay = self: _: {
+  niv-overlay = self: super: {
     niv = self.symlinkJoin {
       name = "niv";
-      paths = [niv];
+      paths = [super.niv];
       buildInputs = [self.makeWrapper];
       postBuild = ''
         wrapProgram $out/bin/niv \

sources.nix

@@ -3,9 +3,7 @@ let
     url = "https://github.com/nmattia/niv/tarball/df49d53b71ad5b6b5847b32e5254924d60703c46";
     sha256 = "1j5p8mi1wi3pdcq0lfb881p97i232si07nb605dl92cjwnira88c";
   };
-  sources = import "${nivSrc}/nix/sources.nix" {
-    sourcesFile = ./sources.json;
-  };
-  niv = import nivSrc {};
 in
-  niv // sources
+  import "${nivSrc}/nix/sources.nix" {
+    sourcesFile = ./sources.json;
+  }