firejail: fixing the 'xdg-dbus-proxy' dependency

xdg-dbus-proxy path is hardcoded in the common.h file in the firejail
source code. if this binary is not found, dbus filtering capabilities
of firejail get limited i.e. you can only entirely disable or entirely
enable dbus communication.

pkgs/os-specific/linux/firejail/default.nix

@@ -1,4 +1,4 @@
-{stdenv, fetchurl, fetchpatch, which, nixosTests}:
+{stdenv, fetchurl, fetchpatch, which, xdg-dbus-proxy, nixosTests}:
 let
   s = # Generated upstream information
   rec {
@@ -25,6 +25,10 @@ stdenv.mkDerivation {
     substituteInPlace etc/firejail.config --replace \
       '# follow-symlink-as-user yes' \
       'follow-symlink-as-user no'
+
+    # Fix the path to 'xdg-dbus-proxy' hardcoded in the 'common.h' file
+    substituteInPlace src/include/common.h \
+      --replace '/usr/bin/xdg-dbus-proxy' '${xdg-dbus-proxy}/bin/xdg-dbus-proxy'
   '';
 
   preConfigure = ''