fix(snap): bump source-tag to v1.2.8 to match version field

The snapcraft.yaml `version:` field was bumped to 1.2.8 but
`source-tag:` under parts.qbz still pointed to v1.2.0. The Snap
Store build farm clones the repo and runs `git checkout v1.2.0`,
so every revision since 1.2.1 was published with metadata "1.2.X"
but a binary compiled from v1.2.0 source — silent regression
visible only when the user opens the app and sees the wrong
version reported.

Affects 8 releases (1.2.1 through 1.2.8). Fixes #361.

cc

snapcraft.yaml

@@ -97,7 +97,7 @@ parts:
   qbz:
     plugin: rust
     source: https://github.com/vicrodh/qbz.git
-    source-tag: v1.2.0
+    source-tag: v1.2.8
     build-packages:
       - libwebkit2gtk-4.1-dev
       - libgtk-3-dev