hold snap revision to prevent refresh (#251)

## Issue
Snaps will auto refresh, meaning that snaps installed inside a charm and
pinned to a specific revision won't necessarily be kept at that
revision. This means a refresh will update the snap, leading to
potential errors within the charm


## Solution
hold the snap revision

src/charm.py

@@ -810,11 +810,11 @@ def _install_snap_packages(self, packages: List[str]) -> None:
             try:
                 snap_cache = snap.SnapCache()
                 snap_package = snap_cache[snap_name]
-
-                if not snap_package.present:
-                    snap_package.ensure(
-                        snap.SnapState.Latest, channel=snap_channel, revision=snap_revision
-                    )
+                snap_package.ensure(
+                    snap.SnapState.Latest, channel=snap_channel, revision=snap_revision
+                )
+                # snaps will auto refresh so it is necessary to hold the current revision
+                snap_package.hold()
 
             except snap.SnapError as e:
                 logger.error(