toolchain-shar-extract.sh: do not allow $ in paths for ext SDK

If you put an $ character in the path, SDK installation fails during the
preparation stage, so add this to the disallowed characters.

Fixes [YOCTO #8625].

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