flake: don't refetch unmodified inputs by recursive follows

Closes #460

I managed to trigger the issue by having the following inputs (shortened):

    authentik-nix.url = "github:nix-community/authentik-nix";
    authentik-nix.inputs.poetry2nix.inputs.nixpkgs.follows = "nixpkgs";

When evaluating this using

    nix-eval-jobs --flake .#hydraJobs

I got the following error:

    error: cannot update unlocked flake input 'authentik-nix/poetry2nix' in pure mode

The issue we have here is that `authentik-nix/poetry2nix` was written
into the `overrideMap` which caused Nix to assume it's a new input and
tried to refetch it (#460) or errored out in pure mode
(nix-eval-jobs / Hydra).

The testcase unfortunately only involves checking for the output log
and makes sure that something *is* logged on the first fetch so that
the test doesn't rot when the logging changes since I didn't
manage to trigger the error above with the reproducer from #460. In
fact, I only managed to trigger the `cannot update unlocked flake input`
error in this context with `nix-eval-jobs`.

Change-Id: Ifd00091eec9a0067ed4bb3e5765a15d027328807

src/libexpr/flake/flake.cc

@@ -342,8 +342,21 @@ static void updateOverrides(std::map<InputPath, FlakeInput> & overrideMap, const
     for (auto & [id, input] : overrides) {
         auto inputPath(inputPathPrefix);
         inputPath.push_back(id);
-        // Do not override existing assignment from outer flake
-        overrideMap.insert({inputPath, input});
+
+        /* Given
+         *
+         * { inputs.hydra.inputs.nix-eval-jobs.inputs.lix.follows = "lix"; }
+         *
+         * then `nix-eval-jobs` doesn't have an override.
+         * It's neither replaced using follows nor by a different
+         * URL. Thus no need to add it to overrides and thus re-fetch
+         * it.
+         */
+        if (input.ref || input.follows) {
+            // Do not override existing assignment from outer flake
+            overrideMap.insert({inputPath, input});
+        }
+
         updateOverrides(overrideMap, input.overrides, inputPath);
     }
 }