mission-planner: reduce closure size

The release zip file doesn't contain a top level directory. The previous
solution of `sourceRoot = "."` followed by an `installPhase` of `mv *
$out/...` does install correctly. However, this grabs detritus from the
Nix build sandbox root, in particular `env-vars`, resulting in an
accidental run-time dependency on many build-time inputs.

Fix by instead using a custom `unpackPhase` to change into an empty
directory, then extracting the release and proceeding with the
installation from there, so that only files from the release are
installed. This reduces closure size by ~425M.

Tested that Mission Planner opens and connects to a drone.

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