Fix update URL for base-browser nightly

The update URLs for base-browser look like this: https://nightlies.tbb.torproject.org/nightly-updates/updates/basebrowser-nightly-linux-x86_64/

However, we currently set it to https://nightlies.tbb.torproject.org/nightly-updates/updates/nightly-basebrowser-linux-x86_64/