chore: nix flake update

Switched to nixpkgs-unstable
Replaced postgresql dependency with much smaller libpq.
Utilise new inputs.self.submodules feature to simplify flake use
Moved apps to a separate file
Changelog-None

nix/apps.nix

@@ -0,0 +1,31 @@
+{ ... }:
+{
+  perSystem =
+    {
+      config,
+      pkgs,
+      system,
+      self',
+      ...
+    }:
+    {
+      apps = {
+        lightningd = {
+          program = "${self'.packages.cln}/bin/lightningd";
+          meta.description = "Core Lightning daemon";
+        };
+        lightning-cli = {
+          program = "${self'.packages.cln}/bin/lightning-cli";
+          meta.description = "Core Lightning command line interface";
+        };
+        lightning-hsmtool = {
+          program = "${self'.packages.cln}/bin/lightning-hsmtool";
+          meta.description = "Core Lightning HSM tool";
+        };
+        reckless = {
+          program = "${self'.packages.cln}/bin/reckless";
+          meta.description = "Core Lightning reckless tool";
+        };
+      };
+    };
+}

nix/pkgs/default.nix

@@ -26,26 +26,27 @@ stdenv.mkDerivation {
   # when building on darwin we need cctools to provide the correct libtool
   # as libwally-core detects the host as darwin and tries to add the -static
   # option to libtool, also we have to add the modified gsed package.
-  nativeBuildInputs =
-    [
-      autoconf
-      autogen
-      automake
-      gettext
-      gitMinimal
-      postgresql
-      libtool
-      lowdown
-      pkgconf
-      py3
-      unzip
-      which
-    ]
-    ++ lib.optionals postgresSupport [ postgresql ]
-    ++ lib.optionals stdenv.isDarwin [
-      cctools
-      darwin.autoSignDarwinBinariesHook
-    ];
+  nativeBuildInputs = [
+    autoconf
+    autogen
+    automake
+    gettext
+    gitMinimal
+    libtool
+    lowdown
+    pkgconf
+    py3
+    unzip
+    which
+  ]
+  ++ lib.optionals postgresSupport [
+    libpq
+    libpq.pg_config
+  ]
+  ++ lib.optionals stdenv.isDarwin [
+    cctools
+    darwin.autoSignDarwinBinariesHook
+  ];
 
   buildInputs = [
     gmp