Integrate fenix toolchain into tor-browser-build's master
We decided to not create a new branch to take care of the Fenix toolchain while continuing to build nightly builds with the ESR 68 toolchains (and later ESR 78 toolchains for desktop builds). Rather, we'll follow boklm's idea of namespacing the projects to fenix-$project if there are Fenix specific needs and keep everything on
master. This should avoid diverging branches and a tricky merge at the end.