GitLab

In a distributed, non-homogeneous CI setup, the binary-toolchain setup
script might not run on the machine that needs the toolchains.  Make
this per-build and it will always be there, at the expense of running on
builds that might not need it (though it still should be fast).

Also, there is an issue with the directory where the binary toolchain is
located being global, and racing against other systems using, setting up,
and tearing down.  Link this to a local directory to avoid any races.
Signed-off-by: Jon Mason <jon.mason@arm.com>