snapcraft: Fix *_ORIG variables

Ensure subiquity-server and ubuntu-desktop-bootstrap apps
have PATH_ORIG, PYTHON_ORIG, and PYTHONPATH_ORIG environment variables.
Lacking these causes usage of subiquity's system_scripts to fail to find
the right python _outside_ of the snap.

Also remove LD_LIBRARY_PATH_ORIG since it's not needed anymore.

snap/snapcraft.yaml

@@ -20,8 +20,8 @@ apps:
     restart-condition: always
     environment:
       PATH_ORIG: $PATH
+      PYTHON_ORIG: $PYTHON
       PYTHONPATH_ORIG: $PYTHONPATH
-      LD_LIBRARY_PATH_ORIG: $LD_LIBRARY_PATH
 
   subiquity-loadkeys:
     command: bin/subiquity/bin/subiquity-loadkeys
@@ -31,6 +31,9 @@ apps:
     command-chain: [bin/launcher]
     desktop: usr/share/applications/ubuntu-desktop-bootstrap.desktop
     environment:
+      PYTHON_ORIG: $PYTHON
+      PYTHONPATH_ORIG: $PYTHONPATH
+      PATH_ORIG: $PATH
       PATH: $SNAP/usr/bin:$SNAP/bin:$PATH
       LIBGL_DRIVERS_PATH: $SNAP/usr/lib/$CRAFT_ARCH_TRIPLET/dri
       GIO_MODULE_DIR: $SNAP/usr/lib/$CRAFT_ARCH_TRIPLET/gio/modules