Fix input-output-hk#824 (input-output-hk#824)

flake.nix

@@ -7,7 +7,7 @@
     # Using the eval-on-build version here as the plan is that
     # `builtins.currentSystem` will not be supported in flakes.
     # https://github.com/NixOS/rfcs/pull/49/files#diff-a5a138ca225433534de8d260f225fe31R429
-    overlay = (self.overlays {}).combined-eval-on-build;
+    overlay = (self.overlays {sourcesOverride = self.sources;}).combined-eval-on-build;
     overlays = import ./overlays;
     config = import ./config.nix;
     # We can't import ./nix/sources.nix directly, because that uses nixpkgs to fetch by default,