snapcraft: set PATH/PYTHON/PYTHONPATH _ORIG

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 python outside of the snap. This will result in blocking installs
since suiquity relies on cloud-init scripts to validate user data.

A special note on PATH_ORIG: For some unknown reason - potentially a
snapcraft bug - when setting environment variables in the subiquity-server
environment, the PATH variable has already been modified to include
paths with a SNAP prefix. This means we can't set PATH_ORIG to PATH
like we do in subiquity. Instead we use the documented default of PATH
for classic confinement snaps in [0] as the value for PATH_ORIG to ensure
we get only paths to outside of the snap.

[0] https://snapcraft.io/docs/environment-variables#heading--path

snap/snapcraft.yaml

@@ -21,7 +21,8 @@ apps:
     environment:
       APPORT_DATA_DIR_ORIG: $APPORT_DATA_DIR
       APPORT_DATA_DIR: $SNAP/usr/share/apport
-      PATH_ORIG: $PATH
+      PATH_ORIG: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
+      PYTHON_ORIG: $PYTHON
       PYTHONPATH_ORIG: $PYTHONPATH
 
   subiquity-loadkeys:
@@ -34,7 +35,10 @@ apps:
     environment:
       APPORT_DATA_DIR_ORIG: $APPORT_DATA_DIR
       APPORT_DATA_DIR: $SNAP/usr/share/apport
+      PATH_ORIG: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
       PATH: $SNAP/usr/bin:$SNAP/bin:$PATH
+      PYTHON_ORIG: $PYTHON
+      PYTHONPATH_ORIG: $PYTHONPATH
       LIBGL_DRIVERS_PATH: $SNAP/usr/lib/$CRAFT_ARCH_TRIPLET/dri
       GIO_MODULE_DIR: $SNAP/usr/lib/$CRAFT_ARCH_TRIPLET/gio/modules
       LIVE_RUN: 1