Remove a possible existing store path when building CA derivations

In case a previous interrupted build left a garbage path laying around,
remove it before trying to move the path to its final location.

Fix #4858

src/libstore/build/local-derivation-goal.cc

@@ -2487,6 +2487,7 @@ void LocalDerivationGoal::registerOutputs()
                 assert(newInfo.ca);
             } else {
                 auto destPath = worker.store.toRealPath(finalDestPath);
+                deletePath(destPath);
                 movePath(actualPath, destPath);
                 actualPath = destPath;
             }