fish: Fix manpage completion generation with paths containing spaces (#6703)

Manual pages with spaces in their paths can lead to argument injection.

tests/modules/programs/fish/default.nix

@@ -4,4 +4,5 @@
   fish-functions = ./functions.nix;
   fish-no-functions = ./no-functions.nix;
   fish-plugins = ./plugins.nix;
+  fish-manpage = ./manpage.nix;
 }

tests/modules/programs/fish/manpage.nix

@@ -0,0 +1,12 @@
+{ lib, pkgs, ... }: {
+  config = {
+    programs.fish = { enable = true; };
+
+    home.packages = [
+      (pkgs.runCommand "manpage-with-space" { } ''
+        mkdir -p $out/share/man/man1
+        echo "It works!" >"$out/share/man/man1/hello -inject.1"
+      '')
+    ];
+  };
+}