Fixes#185095.
Added `gnused` to `smartmontools`.
`/etc/smartd_warning.sh` requires `sed` to be in the path.
Previously failing with:
```
Test of /nix/store/134ckj3riid6ss5iy91b6b14bqyi7qf3-smartd-notify.sh to <nomailer> produced unexpected output (118 bytes) to STDOUT/STDERR:
/nix/store/hhfr3ih96hzgxhhmjxbns726qx6mfff1-smartmontools-7.3/etc/smartd_warning.sh: line 125: sed: command not found
Test of /nix/store/134ckj3riid6ss5iy91b6b14bqyi7qf3-smartd-notify.sh to <nomailer>: failed (32-bit/8-bit exit status: 32512/127)
```