Reformat all projects; switch to overlay format

opa/flake.nix

@@ -6,23 +6,23 @@
     nixpkgs.url = "github:NixOS/nixpkgs";
   };
 
-  outputs = { self, flake-utils, nixpkgs }:
+  outputs =
+    { self
+    , flake-utils
+    , nixpkgs
+    }:
+
     flake-utils.lib.eachDefaultSystem (system:
-      let
-        pkgs = import nixpkgs { inherit system; };
-        inherit (pkgs) conftest mkShell open-policy-agent;
+    let
+      pkgs = import nixpkgs { inherit system; };
+    in
+    {
+      devShell = pkgs.mkShell {
+        buildInputs = with pkgs; [ open-policy-agent conftest ];
 
-        opaTools = [ conftest open-policy-agent ];
-      in
-      {
-        devShells = {
-          default = mkShell {
-            buildInputs = opaTools;
-
-            shellHook = ''
-              ${open-policy-agent}/bin/opa version
-            '';
-          };
-        };
-      });
+        shellHook = ''
+          ${pkgs.open-policy-agent}/bin/opa version
+        '';
+      };
+    });
 }