BaseTools: Fix a bug for ExpandMacros to support mixed case ENV var

BaseTools: Fix a bug for ExpandMacros to support mixed case ENV var

os.environ contains all environment variables uppercase on Windows which
cause the key in the self.MacroDictionary is uppercase, but the real
variable name maybe mixed case, eg:WINSDK81x86, then we can't find the
variable in the self.MacroDictionary.

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>