a99b748931 introduced an environment variable for changing the flatpak path for exports.
On NixOS, we do not have a stable path we could use so we used to patch Flatpak to use `flatpak` program from `PATH`.
With the recent change, we drop our downstream patch in favour of setting the environment variable to `flatpak` but the tests do not expect that.
This is the test part of our former downstream patch so that tests can pass with exports using `flatpak` instead of an absolute path.
It still expects the binary path to end with `flatpak` so arbitrary `$FLATPAK_BINARY` will not work but we do not pass the environment variable to tests so we cannot do much better.
96dbe28cfa
