Add auto-override field to Functions.nix

Modify overridePinnedDeps in Functions.nix to check for auto-override field in source specs.

Current behavior: overridePinnedDeps applies to ALL sources in Sources.json, returning null for packages not in nixpkgs.

New behavior: Only apply to sources where auto-override != false (default true for backward compat).

Implementation:

overridePinnedDeps = builder:
  let
    shouldOverride = name: 
      (super.sources.${name}.auto-override or true) == true;
    overrideable = builtins.filter shouldOverride (builtins.attrNames super.sources);
  in
    super.lib.genAttrs overrideable builder;

Files: Omni/Bild/Functions.nix