fish: escape abbr expansions once again

Commit 8cedd6 `fish: support flexible abbreviations` removed shell
escaping for fish shell abbr values. This was a dangerous breaking
change offered little value and made writing abbr expansions more
difficult. This commit restores automatic shell escaping of fish abbr
values.

modules/programs/fish.nix

@@ -207,7 +207,8 @@ let
       modifiers = if isAttrs def then mods else "";
       expansion = if isAttrs def then def.expansion else def;
     in "abbr --add ${modifiers} -- ${name}"
-    + optionalString (expansion != null) " \"${expansion}\"") cfg.shellAbbrs);
+    + optionalString (expansion != null) " ${escapeShellArg expansion}")
+    cfg.shellAbbrs);
 
   aliasesStr = concatStringsSep "\n"
     (mapAttrsToList (k: v: "alias ${k} ${escapeShellArg v}") cfg.shellAliases);