refactor(flake.nix): externalize pkgs, add overlay

This splits the complexity of the `flake.nix` into multiple files. At
cross-compiled and static builds at the benefit of simpler nix
expressions and generally better cross compilation compatibility.
the same time, naersk is removed; causing much slower builds for cross-
compiled packages.

This partially addresses the points mentioned in #412.

pkgs/rosenpass-oci-image.nix

@@ -0,0 +1,11 @@
+{ dockerTools, buildEnv, rosenpass }:
+
+dockerTools.buildImage {
+  name = rosenpass.name + "-oci";
+  copyToRoot = buildEnv {
+    name = "image-root";
+    paths = [ rosenpass ];
+    pathsToLink = [ "/bin" ];
+  };
+  config.Cmd = [ "/bin/rosenpass" ];
+}