`//labgrid/run` transfers tools to create the exec root to a platform dependant directory

the Runner transfers tools to the $HOME dir on the device to setup the exec root. Under certain situations it is not possible to write under the $HOME dir. We need a way to provide a platform or config dependent place where tools will be transferred.

One implementation would be to supply an env var such as LG_TOOLS to labgrid_config_toolchain, then Runner will read this and set it as the base dir for mktemp.