Fix eval caching for path flakes

This fix is somewhat hacky since the accessor/input fingerprint
situation is a bit of a mess.

src/libfetchers/fetchers.cc

@@ -354,6 +354,8 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
 
     if (!accessor->fingerprint)
         accessor->fingerprint = result.getFingerprint(store);
+    else
+        result.cachedFingerprint = accessor->fingerprint;
 
     return {accessor, std::move(result)};
 }

tests/functional/flakes/flakes.sh

@@ -374,6 +374,7 @@ nix build -o $TEST_ROOT/result git+file://$flakeGitBare
 mkdir -p $flake5Dir
 writeDependentFlake $flake5Dir
 nix flake lock path://$flake5Dir
+[[ "$(nix flake metadata path://$flake5Dir --json | jq -r .fingerprint)" != null ]]
 
 # Test tarball flakes.
 tar cfz $TEST_ROOT/flake.tar.gz -C $TEST_ROOT flake5