multiboot: Allow rewriting grub config without sudo.

Allow the `MULTIBOOT_MNT` environment to be passed in. My usage of this
variable is as follows:

```shell
$ MULTIBOOT_MNT="$(findmnt -n -o target LABEL=multiboot)" ./multiboot-setup grubcfg
```

This allows simple updates of the grub config, while taking advantage of
the auto mounter provided by Gnome.