ci: prefix rolling-channel release tags with "v" for consistency

Stable releases were already "vX.Y.Z" (from the actual git tag pushed for
that channel), but nightly/beta tags were bare "X.Y.Z-nightly"/"X.Y.Z-beta"
— inconsistent with the v-prefixed convention used everywhere else. Every
channel's tag now starts with v: "vX.Y.Z-nightly", "vX.Y.Z-beta", "vX.Y.Z".

.github/workflows/release.yml

@@ -83,7 +83,7 @@ jobs:
           fi
 
           if [ "$CHANNEL" != "stable" ]; then
-            TAG="${BASE_VERSION}-${CHANNEL}"
+            TAG="v${BASE_VERSION}-${CHANNEL}"
           fi
 
           echo "Resolved channel=$CHANNEL tag=$TAG (base version $BASE_VERSION)"