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.

pkgs/by-name/mi/mission-planner/package.nix

@@ -34,7 +34,17 @@ stdenv.mkDerivation rec {
     mono
     unzip
   ];
-  sourceRoot = ".";
+
+  # zip has no outer directory, so make one and unpack there
+  unpackPhase = ''
+    runHook preUnpack
+
+    mkdir -p source
+    cd source
+    unzip -q $src
+
+    runHook postUnpack
+  '';
 
   AOT_FILES = [
     "MissionPlanner.exe"