BaseTools/BuildEnv: override "set -C" (noclobber) in sourcing shell env

BaseTools/BuildEnv: override "set -C" (noclobber) in sourcing shell env

The BuildEnv utility is sourced (executed by the user's interactive shell)
when the user sets up the build session. Some users like to set -C
(noclobber) for some additional safety in their shells, which trips up
BuildEnv. Update the redirection operator so that it overrides noclobber.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>