Fixes #2522

Skip embedding client.alloc_dir when building chroot. If a user
configures a Nomad client agent so that the chroot_env will embed the
client.alloc_dir, Nomad will happily infinitely recurse while building
the chroot until something horrible happens. The best case scenario is
the filesystem's path length limit is hit. The worst case scenario is
disk space is exhausted.

A bad agent configuration will look something like this:

```hcl
data_dir = "/tmp/nomad-badagent"

client {
  enabled = true

  chroot_env {
    # Note that the source matches the data_dir
    "/tmp/nomad-badagent" = "/ohno"
    # ...
  }
}
```

Note that `/ohno/client` (the state_dir) will still be created but not
`/ohno/alloc` (the alloc_dir).
While I cannot think of a good reason why someone would want to embed
Nomad's client (and possibly server) directories in chroots, there
should be no cause for harm. chroots are only built when Nomad runs as
root, and Nomad disables running exec j...