apostrophe: fix HTML export by referencing fira fonts from store

pandoc can embed fonts and stylesheets on export which apostrophe uses —
however it has hardcoded paths for the fonts which only work in the
flatpak version, so we need to substitute them.

pkgs/applications/editors/apostrophe/default.nix

@@ -2,7 +2,7 @@
 , wrapGAppsHook, pkg-config, desktop-file-utils
 , appstream-glib, pythonPackages, glib, gobject-introspection
 , gtk3, webkitgtk, glib-networking, gnome, gspell, texlive
-, shared-mime-info, libhandy
+, shared-mime-info, libhandy, fira
 }:
 
 let
@@ -31,6 +31,12 @@ in stdenv.mkDerivation rec {
     glib-networking libhandy ];
 
   postPatch = ''
+    substituteInPlace data/media/css/web/base.css                                        \
+      --replace 'url("/app/share/fonts/FiraSans-Regular.ttf") format("ttf")'             \
+                'url("${fira}/share/fonts/opentype/FiraSans-Regular.otf") format("otf")' \
+      --replace 'url("/app/share/fonts/FiraMono-Regular.ttf") format("ttf")'             \
+                'url("${fira}/share/fonts/opentype/FiraMono-Regular.otf") format("otf")'
+
     patchShebangs --build build-aux/meson_post_install.py
   '';