Set up nightly builds on tb-build-01.tpo

I think we need to do the following things to set up nightly builds on tb-build-01:

• add nightly build configuration files in tor-browser-bundle-testsuite.git (I opened tor-browser-bundle-testsuite#40015 (closed) for this)
• git clone tor-browser-bundle-testsuite.git on tb-build-01 as the tb-build user
• update TBBTestSuite/TestSuite/TorBrowserBuild.pm locally to not checkout the master branch of tor-browser-build but a specific branch containing the changes for #23631 (closed) and rbm#40001 (closed) (while they are not yet merged).
• add a tools/prune-old-builds script: #40221 (closed)
• add a cron job with the tbb-testsuite command to start nightly builds, and the prune-old-builds command to remove old builds (tools/ansible/roles/tbb-nightly-build/templates/start-tbb-nightly can be used as an example)
• check that nightly builds are built, and emails sent
• Set up rsync between tb-build-01 and tbb-nightlies-master: #40228 (closed)
• Adapt the sign-nightly script to be able to fetch builds from a local directory: #40229 (closed)
• Update TBBTestSuite/TestSuite/RBMBuild.pm to run rsync after each build: tor-browser-bundle-testsuite#40018 (closed)
• merge #23631 (closed) and rbm#40001 (closed)
• remove the local changes to TBBTestSuite/TestSuite/TorBrowserBuild.pm to build the master branch
• update tools/signing/nightly/config.yml to fetch builds from the new URL and update nightly gpg key (#40209 (closed))
• remove tools/ansible/roles/tbb-nightly-build from tor-browser-build: #40244 (closed)

/cc @gk