snapcraft: Fix *_ORIG variables (#842)

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. This is currently causing
installs to fail on daily desktop-builds.

Also remove LD_LIBRARY_PATH_ORIG since it's not needed anymore.

The rational for removing LD_LIBRARY_PATH is mostly that we don't need
it. However, after injecting a new snap to test these changes I found
subiquity struggling to replace LD_LIBRARY_PATH_ORIG because
LD_LIBRARY_PATH wasn't set. LD_LIBRARY_PATH_ORIG is empty, which means
the control flow in the environment cleaner function executes [this
line](https://github.com/canonical/subiquity/blob/a57ed5a5e3ba1fd2212fae3c2f3e84cc8459a158/subiquitycore/utils.py#L73)
which cause a `KeyError` because LD_LIBRARY_PATH is unset (trying to
delete a non-existent key). Strangely this error isn't present in an
unmodified ISO. This leads me to believe that the way I'm building the
snap is slightly different than the launchpad setup. (I can download the
snap and inject it without encountering the issue.) Regardless, we can
remove LD_LIBRARY_PATH_ORIG because we're [always going to remove it
from the cleaned environment
anyways](https://github.com/canonical/subiquity/blob/a57ed5a5e3ba1fd2212fae3c2f3e84cc8459a158/subiquitycore/utils.py#L75).

LP: [#2082851](https://pad.lv/2082851)

snap/snapcraft.yaml

@@ -19,9 +19,11 @@ apps:
     daemon: simple
     restart-condition: always
     environment:
-      PATH_ORIG: $PATH
+      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
+      PYTHON_ORIG: $PYTHON
       PYTHONPATH_ORIG: $PYTHONPATH
-      LD_LIBRARY_PATH_ORIG: $LD_LIBRARY_PATH
 
   subiquity-loadkeys:
     command: bin/subiquity/bin/subiquity-loadkeys
@@ -31,7 +33,12 @@ apps:
     command-chain: [bin/launcher]
     desktop: usr/share/applications/ubuntu-desktop-bootstrap.desktop
     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