ci(release): disable tauri-action updater repack, upload .sig ourselves

Root cause: tauri-action runs a second "Packaging" pass after tauri CLI
finishes, which rewrites src-tauri/target/<target>/release/bundle/macos/
Psysonic.app.tar.gz with different bytes. The .sig produced by tauri CLI
no longer matches the new hash, so tauri-action silently deletes it —
leaving the directory with only .app and .app.tar.gz (no .sig for our
manifest generator to consume).

Fix: pass `includeUpdaterJson: false` to tauri-action so it skips the
repack + updater JSON upload entirely, then copy both the .app.tar.gz
and .app.tar.gz.sig produced by tauri CLI into the workspace root with
the expected asset names and `gh release upload` them.

Also disables build-linux and the Windows matrix entry during testing
to cut iteration time roughly in half.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "Psysonic",
-  "version": "1.34.17",
+  "version": "1.34.18",
   "identifier": "dev.psysonic.player",
   "build": {
     "beforeDevCommand": "npm run dev",