Move checks to project.ncl

It's unclear if we need to implement runCommand ourselves, it's quite
simple to achieve necesary result with plaing NixpkgsPkg override.

Introduce NixEnvironmentVariable contract to handle all possible things
than might end up in `env`.

Part of #58.

flake.nix

@@ -100,31 +100,6 @@
               program = pkgs.lib.getExe testScript;
             };
           };
-
-        checks.alejandra = pkgs.runCommand "check-alejandra" {} ''
-          ${pkgs.lib.getExe pkgs.alejandra} --check ${self}
-          touch $out
-        '';
-
-        checks.nickel-format =
-          pkgs.runCommand "check-nickel-format" {
-            buildInputs = [
-              inputs.nickel.packages.${system}.nickel-lang-cli
-            ];
-          } ''
-            cd ${self}
-            failed=""
-            for f in $(find . -name future -prune -or -name '*.ncl' -print); do
-              if ! diff -u "$f" <(nickel format -f "$f"); then
-                failed="$failed $f"
-              fi
-            done
-            if [ "$failed" != "" ]; then
-              echo "Following files need to be formatted: $failed"
-              exit 1
-            fi
-            touch $out
-          '';
       }
     );
 }

lib/builders.ncl

@@ -1,4 +1,4 @@
-let { NickelDerivation, Derivation, NixString, .. } = import "contracts.ncl" in
+let { NickelDerivation, Derivation, NixString, NixEnvironmentVariable, .. } = import "contracts.ncl" in
 
 let lib = import "lib.ncl" in
 
@@ -56,7 +56,7 @@ in
         },
         structured_env = {},
         env = {
-          stdenv = lib.import_nix "nixpkgs#stdenv"
+          stdenv | default = lib.import_nix "nixpkgs#stdenv",
         },
         nix_drv = env & structured_env,
       }

project.ncl

@@ -15,5 +15,45 @@ let import_nix = organist.lib.import_nix in
         nls = import_nix "nickel#lsp-nls",
       },
     },
+
+  flake.checks
+    | {
+      _ | {
+        version
+        = "",
+        env.stdenv
+        = import_nix "nixpkgs#stdenvNoCC",
+        ..
+      }
+    }
+    | { _ | organist.builders.NixpkgsPkg }
+    = {
+      alejandra = {
+        name = "check-alejandra",
+        env.buildCommand = nix-s%"
+          %{import_nix "nixpkgs#alejandra"}/bin/alejandra --check %{import_nix "self"}
+          touch $out
+        "%,
+      },
+
+      nickel-format = {
+        name = "check-nickel-format",
+        env.buildInputs.nickel = import_nix "nickel#nickel-lang-cli",
+        env.buildCommand = nix-s%"
+          cd %{import_nix "self"}
+          failed=""
+          for f in $(find . -name future -prune -or -name '*.ncl' -print); do
+            if ! diff -u "$f" <(nickel format -f "$f"); then
+              failed="$failed $f"
+            fi
+          done
+          if [ "$failed" != "" ]; then
+            echo "Following files need to be formatted: $failed"
+            exit 1
+          fi
+          touch $out
+        "%,
+      },
+    },
 }
   | organist.contracts.OrganistExpression