Fix unsigned builds (PRs in CI)

These were broken by the interaction of
electron-userland/electron-builder#1401
and our custom unsigned build config.

Author: pimterry

package.json

@@ -30,6 +30,9 @@
     "appId": "tech.httptoolkit.desktop",
     "extends": null,
     "productName": "HTTP Toolkit",
+    "directories": {
+      "buildResources": "build-resources"
+    },
     "extraMetadata": {
       "name": "httptoolkit",
       "productName": "httptoolkit",