Update nightly build gpg key and sign-nightly configuration

We should create a new gpg key on tb-build-01 to sign the nightly builds.

When we are ready to switch to the new nightly build machine, we should add the new key to tor-browser-build/keyring/torbrowser-nightly.gpg and update builds_url in tor-browser-build/tools/signing/nightly/config.yml to fetch builds from the new URL.