toolchain-shar-extract.sh: Ensure it's ran in clean environment

Fixes [YOCTO #8698] -- https://bugzilla.yoctoproject.org/show_bug.cgi?id=8698



If prior to running the toolchain installation script a toolchain
environment script is sourced then the toolchain installation will
fail. This because the environment is now set for the sourced toolchain
and doesn't suit the installation. In particular PATH points to the
toolchain executables.

The fix makes the script recursively call itself a second time with a
clean environment.

Tested by sourcing a previous successfully installed environment, erasing the
previous installation directory and then reinstalling in the same directory.

Signed-off-by: George Nita <george.nita@enea.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>