Let workflow_dispatch be a dry run for the release pipeline

Manual triggers now build all platform prebuilds and list them, but skip
npm publish and GitHub release creation. This gives us a safe way to
verify the full build matrix before tagging a real release.

Tag pushes still do the full release as before.

Author: oetr

.github/workflows/release.yaml

@@ -41,8 +41,23 @@ jobs:
           path: packages/fuzzer/prebuilds
           if-no-files-found: error
 
-  release:
-    name: release
+  verify-prebuilds:
+    name: verify prebuilds
+    needs: [prebuilds]
+    runs-on: ubuntu-latest
+    steps:
+      - name: download prebuilds
+        uses: actions/download-artifact@v3
+        with:
+          name: native-addon
+          path: prebuilds
+          if-no-files-found: error
+      - name: list prebuilds
+        run: ls -lh prebuilds/
+
+  publish:
+    name: publish
+    if: startsWith(github.ref, 'refs/tags/')
     needs: [prebuilds]
     runs-on: ubuntu-latest
     permissions: