nixos/boot.plymouth.font: escape font path

This fixes the usage of fonts whose filenames contain special
characters of various sorts.

For example, the setting
```nix
boot.plymouth.font =
  "${pkgs.noto-fonts}/share/fonts/noto/NotoSans[wdth,wght].ttf";
```

will cause a build failure, without this patch.

nixos/modules/system/boot/plymouth.nix

@@ -219,7 +219,7 @@ in
         # Fonts
         "/etc/plymouth/fonts".source = pkgs.runCommand "plymouth-initrd-fonts" {} ''
           mkdir -p $out
-          cp ${cfg.font} $out
+          cp ${escapeShellArg cfg.font} $out
         '';
         "/etc/fonts/fonts.conf".text = ''
           <?xml version="1.0"?>