Tools/Fsp: Replace PYTHON_HOME with sys.executable

Currently script Python interpreter is inconsistent with the
interpreter specified by Python home. This patch is to change
pythonhome to sys.executable.

Signed-off-by: MingYue Liang <mingyuex.liang@intel.com>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>