Move nightly builds to tb-build-06

With tpo/tpa/team#40984 (closed) we have a new machine for nightly build.

While moving nightly builds there, we should configure the build to use /tmp during the build.

List of tasks:

• Update references to tb-build-01 on wiki page: https://gitlab.torproject.org/tpo/applications/tor-browser/-/wikis/Nightly_Builds_Setup
• Update references to tb-build-01 in tor-browser-bundle-testsuite.git: !33 (merged)
• Use /tmp in rbm.local.conf: !34 (merged)
• Generate two new ssh key on tb-build-06 for user tb-builder, and add them to tbb-nightlies-master (with command option, following https://gitlab.torproject.org/tpo/applications/tor-browser/-/wikis/Nightly_Builds_Setup#rsync-between-tb-build-06-and-tbb-nightlies-master)
• Remove old ssh keys from /etc/ssh/userkeys/tbb-nightlies on tbb-nightlies-master
• Copy gpg key (CB91EDC56B9E276D7144ABD567509BB73B8DE954) from tb-build-01 to tb-build-06
• Copy /home/tb-builder/nightly-builds from tb-build-01 to tb-build-06
• Copy /home/tb-builder/tor-browser-builds-archive from tb-build-01 to tb-build-06
• Clone /home/tb-builder/tor-browser-bundle-testsuite on tb-build-06
• Disable cron job on tb-build-01
• Add cron job on tb-build-06