Fix assertion failure when a path is locked

Fixes:

  nix-store: src/libstore/build.cc:3649: void nix::Worker::run(const Goals&): Assertion `!awake.empty()' failed.

src/libstore/pathlocks.cc

@@ -136,6 +136,7 @@ bool PathLocks::lockPaths(const PathSet & _paths,
                         /* Failed to lock this path; release all other
                            locks. */
                         unlock();
+                        lockedPaths_.lock()->erase(lockPath);
                         return false;
                     }
                 }