Add nixpkgs-fmt/naersk

This adds a recent nixpkgs-fmt and naersk which is required for the
build.

nix/overlays/nixpkgs-overrides.nix

@@ -1,7 +1,9 @@
 self: super: {
+  naersk = super.callPackage ((import ../sources.nix).naersk) { };
+  nixpkgs-fmt = super.callPackage ((import ../sources.nix).nixpkgs-fmt) { };
   nix-gitignore = super.callPackage ((import ../sources.nix).nix-gitignore) { };
   # We can *nearly* replace this with upstream nixpkgs, but unfortunately we also need a patch
   # that hasn't been merged upstream yet. And you can't override the pieces of a 'bundlerApp'.
-  asciidoctor= super.callPackage ../asciidoctor { };
-  z3 = super.callPackage ../z3.nix {};
+  asciidoctor = super.callPackage ../asciidoctor { };
+  z3 = super.callPackage ../z3.nix { };
 }

nix/sources.json

@@ -48,6 +48,18 @@
         "url": "https://github.com/input-output-hk/iohk-nix/archive/24ddc8f531f320c9e2394d205d0df39857222901.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
+    "naersk": {
+        "branch": "master",
+        "description": "Build rust crates in Nix. No configuration, no code generation, no IFD. Sandbox friendly.",
+        "homepage": "",
+        "owner": "nmattia",
+        "repo": "naersk",
+        "rev": "22b96210b2433228d42bce460f3befbdcfde7520",
+        "sha256": "0qa8jnw71qk1i1fgpydx1nv17cnvlydb9jhd6f7gvhglagm19b4v",
+        "type": "tarball",
+        "url": "https://github.com/nmattia/naersk/archive/22b96210b2433228d42bce460f3befbdcfde7520.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "nix-gitignore": {
         "branch": "master",
         "builtin": false,
@@ -74,6 +86,18 @@
         "url": "https://github.com/NixOS/nixpkgs/archive/db31e48c5c8d99dcaf4e5883a96181f6ac4ad6f6.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
+    "nixpkgs-fmt": {
+        "branch": "master",
+        "description": "Nix code formatter for nixpkgs [maintainer=@zimbatm]",
+        "homepage": "https://nix-community.github.io/nixpkgs-fmt/",
+        "owner": "nix-community",
+        "repo": "nixpkgs-fmt",
+        "rev": "6d94c33c2429dbf173047d688a8c1433e7a712f6",
+        "sha256": "0xx1qcs6zdw3xz3i44gff3qv86fqfg2s1lg9h1vl8b4lkf5yvwg4",
+        "type": "tarball",
+        "url": "https://github.com/nix-community/nixpkgs-fmt/archive/6d94c33c2429dbf173047d688a8c1433e7a712f6.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "nodejs-headers": {
         "builtin": false,
         "rev": "v10.9.0",