Fix nix flake build and check this on CI (#145)

Broken by #144, which made
dependencies incompatible with the pinned rust toolchain.

This PR will check Nix Flake build on CI, but won't block PR merges if
the check fails.

.github/workflows/ci.yml

@@ -72,6 +72,21 @@ jobs:
 
       - name: Check semver
         uses: obi1kenobi/cargo-semver-checks-action@v2
+
+  flake:
+    runs-on: ubuntu-latest 
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v3
+
+      - name: Install nix
+        uses: DeterminateSystems/nix-installer-action@v12
+
+      - name: Cache nix dependencies
+        uses: DeterminateSystems/magic-nix-cache-action@v7
+
+      - name: Build
+        run: nix flake check
 
   update-release-draft:
     runs-on: ubuntu-latest

flake.nix

@@ -1,41 +1,54 @@
 {
-    description = "Protofetch - A source dependency management tool for Protobuf files";
+  description = "Protofetch - A source dependency management tool for Protobuf files";
 
-    inputs = {
-        nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
-        flake-utils.url = "github:numtide/flake-utils";
-        crane = { url = "github:ipetkov/crane"; inputs.nixpkgs.follows = "nixpkgs"; };
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    crane = {
+      url = "github:ipetkov/crane";
+      inputs.nixpkgs.follows = "nixpkgs";
     };
+  };
 
-    outputs = { self, nixpkgs, flake-utils, crane }: flake-utils.lib.eachDefaultSystem (system:
-    let
-        pkgs = import nixpkgs { inherit system; };
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+      crane,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
         inherit (pkgs) lib;
-        craneLib = crane.lib.${system};
-    in {
+        craneLib = crane.mkLib pkgs;
+
+        protofetch = craneLib.buildPackage {
+          pname = "protofetch";
+          src = lib.cleanSourceWith {
+            src = ./.; # The original, unfiltered source
+            filter = path: type: (lib.hasSuffix "\.proto" path) || (craneLib.filterCargoSources path type);
+          };
+          buildInputs = [
+            pkgs.openssl
+            pkgs.libgit2
+            pkgs.pkg-config
+          ] ++ lib.optionals pkgs.stdenv.isDarwin [ pkgs.darwin.apple_sdk.frameworks.Security ];
+          preBuild = ''
+            export HOME=$(mktemp -d)
+          '';
+        };
+      in
+      {
         packages = rec {
-            default = protofetch;
-            protofetch = craneLib.buildPackage {
-                pname = "protofetch";
-                src = lib.cleanSourceWith {
-                    src = ./.; # The original, unfiltered source
-                    filter = path: type:
-                        (lib.hasSuffix "\.proto" path) ||
-                        (craneLib.filterCargoSources path type);
-                };
-                buildInputs = with pkgs; [
-                    openssl
-                    libgit2
-                    pkg-config
-                ] ++ (if stdenv.isDarwin then [darwin.apple_sdk.frameworks.Security] else []);
-                preBuild = ''
-                    export HOME=$(mktemp -d)
-                '';
-            };
+          inherit protofetch;
+          default = protofetch;
         };
-    });
+        checks = {
+          # Build the crate as part of `nix flake check`
+          inherit protofetch;
+        };
+      }
+    );
 }
-
-
-
-