remove incorrect skip if path exists

the path might exists also the build failed

buildbot_nix/buildbot_nix.py

@@ -209,12 +209,6 @@ class NixBuildCommand(buildstep.ShellMixin, steps.BuildStep):
             log: Log = yield self.addLog("nix_error")
             log.addStderr(f"{attr} failed to evaluate:\n{error}")
             return util.FAILURE
-        path = self.getProperty("out_path")
-
-        # FIXME: actually we should check if it exists in the remote machine
-        if os.path.exists(path):
-            # build already succeeded
-            return util.SKIPPED
 
         # run `nix build`
         cmd: remotecommand.RemoteCommand = yield self.makeRemoteShellCommand()