Use tor-browser-build/tmp rather than /tmp as default tmp directory
We are currently using /tmp as the default temporary directory, however it seems to be quite common that this directory is mounted on a small tmpfs partition, which makes the build fail because of lack of disk space.
To avoid this, we should instead use a tmp directory inside the tor-browser-build
directory by default.
https://trac.torproject.org/projects/tor/ticket/23382#comment:1