Sign mullvad-browser nightly updates

After enabling mullvad-browser nightly builds in tor-browser-bundle-testsuite#40076 (closed), we should enable signing of mullvad-browser nightly updates, by updating tools/signing/nightly/config.yml, and checking out the new commit on tbb-nightlies@tbb-nightlies-master.torproject.org, after creating a new key there using tools/signing/nightly/create-nightly-mar-signing-key.