nsenter: Do not enter user namespace

Running `nsenter` with `-U` fails:

```
nsenter: setns(): can't reassociate to namespace 'user': Invalid argument
```

This is probably because the container runs in the same user namespace
as the host anyway, so it really doesn't matter.

nsenter.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-exec nsenter -m -u -i -n -p -U -t 1 "${0##*/}" "$@"
+exec nsenter -m -u -i -n -p -t 1 "${0##*/}" "$@"