flake.nix: Only add _file-key if position of args.modules is actually known to the evaluator

This happens if the evaluator "loses" the position of an
attr-declaration[1] because of e.g. too many nested function-calls to
build the final attr-set.

While the actual issue should be fixed in Nix itself, this is IMHO a
fair workaround to unblock affected users[2].

[1] https://github.com/NixOS/nixpkgs/commit/e14c24593420bb9057e7f38b40d17137eaeff9dd#commitcomment-53645936
[2] It seems as everyone using `divnix/digga` or `flake-utils-plus`
    are affected:
    * https://github.com/divnix/digga/issues/87

flake.nix

@@ -49,12 +49,18 @@
                   })).config;
 
                 moduleDeclarationFile =
-                  (builtins.unsafeGetAttrPos "modules" args).file;
+                  let
+                    # Even though `modules` is a mandatory argument for `nixosSystem`, it doesn't
+                    # mean that the evaluator always keeps track of its position. If there
+                    # are too many levels of indirection, the position gets lost at some point.
+                    intermediatePos = builtins.unsafeGetAttrPos "modules" args;
+                  in
+                    if intermediatePos == null then null else intermediatePos.file;
 
                 # Add the invoking file as error message location for modules
                 # that don't have their own locations; presumably inline modules.
                 addModuleDeclarationFile =
-                  m: {
+                  m: if moduleDeclarationFile == null then m else {
                     _file = moduleDeclarationFile;
                     imports = [ m ];
                   };