build: stop producing a disk image

"targets": "all" built a 90MB DMG on every single build, for nobody:
this is installed to /Applications by copying the .app. Building only
the app also shortens each build.

A full rebuild from an empty target directory takes 78 seconds, so the
cache is worth clearing when it grows rather than hoarding.

src-tauri/tauri.conf.json

@@ -36,7 +36,7 @@
   },
   "bundle": {
     "active": true,
-    "targets": "all",
+    "targets": ["app"],
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",