store: guess the URL of failing fixed-output derivations

This is a shameless layering violation in favour of UX. It falls back
trivially to "unknown", so it's purely a UX feature.

Diagnostic sample:

```
error: hash mismatch in fixed-output derivation '/nix/store/sjfw324j4533lwnpmr5z4icpb85r63ai-x1.drv':
        likely URL: https://meow.puppy.forge/puppy.tar.gz
         specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
            got:    sha256-a1Qvp3FOOkWpL9kFHgugU1ok5UtRPSu+NwCZKbbaEro=
```

Change-Id: I873eedcf7984ab23f57a6754be00232b5cb5b02c

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 968f669ec..1dd2ad8aa 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2546,9 +2546,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                     /* Throw an error after registering the path as
                        valid. */
                     worker.hashMismatch = true;
+                    // XXX: shameless layering violation hack that makes the hash mismatch error at least not utterly worthless
+                    auto guessedUrl = getOr(drv->env, "urls", getOr(drv->env, "url", "(unknown)"));
                     delayedException = std::make_exception_ptr(
-                        BuildError("hash mismatch in fixed-output derivation '%s':\n  specified: %s\n     got:    %s",
+                        BuildError("hash mismatch in fixed-output derivation '%s':\n likely URL: %s\n  specified: %s\n     got:    %s",
                             worker.store.printStorePath(drvPath),
+                            guessedUrl,
                             wanted.to_string(SRI, true),
                             got.to_string(SRI, true)));
                 }