display-managers module: improve variable quoting

Fixes #20713, though I'm certain nixpkgs contains loads of places
without proper quoting, as (ba)sh unfortunately encourages that.

The only plus side is that most of such problems in nixpkgs aren't
actually security problems but mere annoyance to those who are foolish
enough to use "weird" characters in critical names.

nixos/modules/services/x11/display-managers/default.nix

@@ -82,12 +82,12 @@ let
 
       # Speed up application start by 50-150ms according to
       # http://kdemonkey.blogspot.nl/2008/04/magic-trick.html
-      rm -rf $HOME/.compose-cache
-      mkdir $HOME/.compose-cache
+      rm -rf "$HOME/.compose-cache"
+      mkdir "$HOME/.compose-cache"
 
       # Work around KDE errors when a user first logs in and
       # .local/share doesn't exist yet.
-      mkdir -p $HOME/.local/share
+      mkdir -p "$HOME/.local/share"
 
       unset _DID_SYSTEMD_CAT
 
@@ -148,7 +148,7 @@ let
       allowSubstitutes = false;
     }
     ''
-      mkdir -p $out
+      mkdir -p "$out"
       ${concatMapStrings (n: ''
         cat - > "$out/${n}.desktop" << EODESKTOP
         [Desktop Entry]