flatpak: fix flatpak-spawn --clear-env

Chromium Flatpak package uses flatpak-spawn command for sandboxing
the child processes. The command comes from flatpak-xdg-utils, which is
presumably included in Flatpak runtimes. The command then calls Spawn
method of the portal.

The portal supports running commands in a clear environment when passed
FLATPAK_SPAWN_FLAGS_CLEAR_ENV flag. Unfortunately, that also clears PATH,
which is probably what prevents `flatpak` command itself from being found.

There is a relevant TODO note in the code:
https://github.com/flatpak/flatpak/blob/1.10.2/portal/flatpak-portal.c#L995-L999

For now, let’s hardcode the path to the binary.

pkgs/development/libraries/flatpak/default.nix

@@ -1,5 +1,6 @@
 { lib, stdenv
 , fetchurl
+, fetchpatch
 , autoreconfHook
 , docbook_xml_dtd_45
 , docbook-xsl-nons
@@ -93,6 +94,15 @@ stdenv.mkDerivation rec {
     # https://github.com/NixOS/nixpkgs/issues/43581
     ./use-flatpak-from-path.patch
 
+    # Hardcode flatpak binary path for flatpak-spawn.
+    # When calling the portal’s Spawn command with FLATPAK_SPAWN_FLAGS_CLEAR_ENV flag,
+    # it will clear environment, including PATH, making the flatpak run fail.
+    # https://github.com/flatpak/flatpak/pull/4174
+    (fetchpatch {
+      url = "https://github.com/flatpak/flatpak/commit/495449daf6d3c072519a36c9e4bc6cc1da4d31db.patch";
+      sha256 = "gOX/sGupAE7Yg3MVrMhFXzWHpFn+izVyjtkuPzIckuY=";
+    })
+
     # Nix environment hacks should not leak into the apps.
     # https://github.com/NixOS/nixpkgs/issues/53441
     ./unset-env-vars.patch