BaseTools/BuildEnv: Do not modify the env 'PACKAGES_PATH' in BuildEnv

BaseTools/BuildEnv: Do not modify the env 'PACKAGES_PATH' in BuildEnv

https://bugzilla.tianocore.org/show_bug.cgi?id=236

The script 'BuildEnv' modifies the value of the environment variable
'PACKAGES_PATH' (line 44). The script will substitute the ':' symbol
(separating multiple paths) with a space.

This is not supposed to happen since users might later use 'PACKAGES_PATH'
during the code-building process under a multiple-workspace scenario.

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