BaseTools: use shutil.copyfile instead shutil.copy2

In Split tool, the copy file actions only need to
copy file content but not need to copy file metadata.

copy2() copies the file metadata that causes split
unit test failed under edk2-basetools CI environment.

So this patch changes the call of copy2() to copyfile().

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