BaseTools: Fix a bug for tooldef class not include the newly Env

BaseTools: Fix a bug for tooldef class not include the newly Env

Prebuild script may update os.environ, but the tooldef class not include
the new ENV variables. so after the Launch prebuild script, we should
re-init the tooldef class to include the new ENV variables.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>