allow forcing use of portal with env var

In some environments, the standard method of opening a program may not work due to e.g. lack of availability of the binaries or presence of conflicting libraries in LD_LIBRARY_PATH. One case of this is currently handled for Flatpak, but others are not. To give users an easy workaround when using portals is desirable, it would be useful to allow overriding the open method choice via an environment variable.

Currently, this could only be done by creating a file, $XDG_RUNTIME_DIR/flatpak-info. An env var such as XDG_OVERRIDE_DE (for all utils) or XDG_OPEN_OVERRIDE_DE would make this simpler. I propose solving it this way rather than just adding an override to force using the portal because it would also cover other cases where a user wants different behavior.

An example of this being done by a packager via a patch adding an env var can be seen in https://github.com/NixOS/nixpkgs/pull/197118