Use /usr/bin/env bash in shebang for portability

Since there's no `/bin/bash` on FreeBSD, it's preferable to use `/usr/bin/env bash` so `/usr/local/bin/bash` is used instead.

You might also need to set the `PATH` variable in the Munin plugin configuration file, depending on how your environment is set up.

```
[nfs*]
env.PATH /usr/bin:/usr/local/bin
```