Issue title: bug: xscreensaver: quirks and packages not found? 
 Issue body: ### Are you following the right branch?

- [X] My Nixpkgs and Home Manager versions are in sync

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Issue description

### Configuration & quirks

I've configured xscreensaver like so:

``` nix 
{ config, pkgs, ... }:
let sys = import ../../system-specific.nix;
in {

  ###########
  # !!!!!!! #
  # WARNING #
  # !!!!!!! #
  ###########

  # There's a quirk: https://www.reddit.com/r/NixOS/comments/rcxpsl/comment/ho1tva1/?utm_source=reddit&utm_medium=web2x&context=3
  # Every time settings are changed, you need to run `xscreensaver-settings` to create the file in ~. 

  services.xscreensaver = {
    enable = true;
    settings = {
      timeout = "0:01:00"; # works correctly 
      mode = "blank";
      dpmsEnabled = true; # doesn't work, screen doesn't blank on timeout (below)
      dpmsOff = "0:01:10"; # doesn't work. 
    };
  };
}
```

I've highlighted some quirks with working with xscreensaver in my comments. I think it would be quite nice to have this highlighted in the docs, if confirmed to be correct. 

### The bug(?)

The main issue, however, is that I see journald entries like these:
```
Aug 14 12:14:52 quasar-nixos-tr xscreensaver[10413]: sh: line 1: xscreensaver-command: command not found
Aug 14 12:14:52 quasar-nixos-tr xscreensaver[2459]: xscreensaver-systemd: 12:14:52: exec: "xscreensaver-command -quiet -deactivate" exited with status 127
^G"
```

Which means that the systemd service cannot find the binaries it needs. Its surprising because [this line](https://github.com/nix-community/home-manager/blob/8675cfa549e1240c9d2abb1c878bc427eefcf926/modules/services/xscreensaver.nix#L38) should ensure that never happens. 

The latter line appears in the logs every time I play a video on youtube, for example, as the dbus based message is passed to xscreensaver to be disabled during media playback (I assume). 


### What I've tried so far

1. Installing xscreensaver in pkgs. 
2. Logging out and back in. 
3. Rebooting. 

None of this has helped. 

### How `home-manager` is installed. 

My `configuration.nix` has a line like:

``` nix 
  home-manager = builtins.fetchTarball "https://github.com/nix-community/home-manager/archive/release-21.11.tar.gz";
```

Which avoids the use of channels. 



### Maintainer CC

@rycee (https://github.com/nix-community/home-manager/blob/8675cfa549e1240c9d2abb1c878bc427eefcf926/modules/services/xscreensaver.nix#L10)

### System information

```markdown
- system: `"x86_64-linux"`
 - host os: `Linux 5.15.50, NixOS, 21.11 (Porcupine)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.16`
 - channels(root): `"nixos-21.11.337971.a1fe662eb26, oldpkgs-19.03.173676.d1dff0bcd9f"`
 - channels(ashesh): `"nixos-20.09.3975.17b101e29df"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`
```

 Comments: 
 Comment 0: I've since upgraded to 22.05 and the issue seems to have disappeared. 
My apologies. Closing but it can be reopened by the maintainers if needed. 
{'source': 'GithubIssues', 'repo_name_with_owner': 'nix-community/home-manager', 'issue_id': 1338200342, 'repo_stars': 1880.0}