Upgrade flake build process

The previous build process was MASSIVELY inefficient in multiple
ways. This is a massive improvement.

flake.nix

@@ -14,21 +14,39 @@
     flake-utils.lib.eachDefaultSystem (system:
       let pkgs = import nixpkgs { inherit system; };
           craneLib = crane.lib.${system};
+
+          commonArgs = {
+            version = "0.1.0";
+            src = craneLib.path ./.;
+            cargoBuildFlags = "-p cli";
+            buildInputs = [ pkgs.openssl ];
+            nativeBuildInputs = [ pkgs.pkg-config ];
+          };
+
+          # Cargo build dependencies/artifacts only
+          # This derivation exists so that multiple builds can reuse
+          # the same build artifacts
+          cargoArtifacts = craneLib.buildDepsOnly (commonArgs // {
+            pname = "ggelo-deps";
+          });
+
+          # Run clippy (and deny all warnings) on the crate source
+          runClippy = craneLib.cargoClippy (commonArgs // {
+            pname = "ggelo-clippy-check";
+            cargoClippyExtraArgs = "--all-targets -- --deny warnings";
+            inherit cargoArtifacts;
+          });
+
+          ggelo = craneLib.buildPackage (commonArgs // {
+            pname = "ggelo";
+            inherit cargoArtifacts;
+          });
       in {
-        packages.ggelo = craneLib.buildPackage {
+        packages.ggelo = ggelo;
-          pname = "ggelo";
+        packages.default = ggelo;
-          version = "0.1.0";
-          src = ./.;
-          cargoBuildFlags = "-p cli";
 
-          nativeBuildInputs = [ pkgs.pkg-config ];
+        checks.build = ggelo;
-          PKG_CONFIG_PATH = "${pkgs.openssl.dev}/lib/pkgconfig";
+        checks.runClippy = runClippy;
-
-          doCheck = true;
-        };
-
-        packages.default = self.packages.${system}.ggelo;
-        checks.default = self.packages.${system}.ggelo;
 
         devShells.default = pkgs.mkShell {
           packages = with pkgs; [ rustc cargo pkg-config rust-analyzer ];