Revert "GH Actions: avoid running node vendor_build.js twice"

This reverts commit b06c034.

After testing, it turned out that the `node vendor_build.js` after `npm
install <package-spec>...` is actually necessary (otherwise the `npm
install <package-spec>...` command has no effect on the resulting bundle
in the `out/` directory), and removing it was a mistake.

.github/workflows/main.yml

@@ -40,6 +40,7 @@ jobs:
       - name: Use stable versions of KSC and JS runtime
         run: |
           npm install kaitai-struct-compiler@latest kaitai-struct@latest
+          node vendor_build.js
         if: github.ref == 'refs/heads/stable'
       - name: Use devel versions of KSC and JS runtime
         run: |