toolchain-shar-extract.sh: allow TERM through into SDK installer env

toolchain-shar-extract.sh: allow TERM through into SDK installer env

In order to show the task progress during the extensible SDK
installation, knotty needs to know it is using a terminal it can
support the "interactive" mode on (i.e. ncurses can be used), which
means it needs access to the TERM variable value, so allow that
through into the cleaned environment within the SDK installation
script.

Part of the implementation for [YOCTO #9613].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>