Tweak how python packages are installed for backup app

per some issues i encountered with pip 20.3.3 and its "new resolver" although
not totally clear what fixed it... :/

rattail_fabric2/backup.py

@@ -127,13 +127,13 @@ def deploy_backup_app(c, deploy, envname, mkvirtualenv=True, user='rattail',
                 'rattail[backup]',
             ]
         else:
-            # these should be same as rattail[backup]
+            # TODO: these should be same as rattail[backup]
             packages = [
                 'msgpack',
-                'borgbackup',
-                'llfuse==1.3.4',
+                'borgbackup[fuse]',
             ]
-        c.sudo("bash -l -c 'workon {} && cdvirtualenv && bin/pip install {}'".format(envname, ' '.join(packages)), user=user)
+        c.sudo("bash -c 'PIP_CONFIG_FILE={0}/pip.conf {0}/bin/pip install {1}'".format(envpath, ' '.join(packages)),
+               user=user)
         if link_borg_to_bin:
             c.sudo("ln -sf {}/bin/borg /usr/local/bin/borg".format(envpath))