Split xeon specific configuration from base

To accomodate the raccoon knights workstation, some of the configuration
pulled by m/common/main.nix has to be removed. To solve it, the xeon
specific parts are placed into m/common/xeon.nix and only the common
configuration is at m/common/base.nix.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/common/base/nix.nix

@@ -0,0 +1,42 @@
+{ pkgs, nixpkgs, bscpkgs, theFlake,  ... }:
+
+{
+  nixpkgs.overlays = [
+    bscpkgs.bscOverlay
+    (import ../../../pkgs/overlay.nix)
+  ];
+
+  nix = {
+    nixPath = [
+      "nixpkgs=${nixpkgs}"
+      "jungle=${theFlake.outPath}"
+    ];
+
+    registry = {
+      nixpkgs.flake = nixpkgs;
+      jungle.flake = theFlake;
+    };
+
+    settings = {
+      experimental-features = [ "nix-command" "flakes" ];
+      sandbox = "relaxed";
+      trusted-users = [ "@wheel" ];
+      flake-registry = pkgs.writeText "global-registry.json"
+        ''{"flakes":[],"version":2}'';
+    };
+
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 30d";
+    };
+  };
+
+  # This value determines the NixOS release from which the default
+  # settings for stateful data, like file locations and database versions
+  # on your system were taken. It‘s perfectly fine and recommended to leave
+  # this value at the release version of the first install of this system.
+  # Before changing this value read the documentation for this option
+  # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
+  system.stateVersion = "22.11"; # Did you read the comment?
+}