flatpak: fix config location

By default the build system sets the config location to
$prefix/etc/flatpak, which(due to the immutability of the nix store)
prevents the user from changing any configuration options.

This moves the config file to /etc/flatpak.

pkgs/development/libraries/flatpak/default.nix

@@ -165,7 +165,9 @@ stdenv.mkDerivation (finalAttrs: {
     "--with-system-bubblewrap=${bubblewrap}/bin/bwrap"
     "--with-system-dbus-proxy=${xdg-dbus-proxy}/bin/xdg-dbus-proxy"
     "--with-dbus-config-dir=${placeholder "out"}/share/dbus-1/system.d"
+    "--with-profile-dir=${placeholder "out"}/etc/profile.d"
     "--localstatedir=/var"
+    "--sysconfdir=/etc"
     "--enable-gtk-doc"
     "--enable-installed-tests"
   ];