BaseTools: Fix the issue in VS prefix setting for VS2017/VS2019

BaseTools: Fix the issue in VS prefix setting for VS2017/VS2019

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2896

When VS2017/VS2019 are both installed. VS prefix setting will
wrongly be set. VS2017_PREFIX is set to the same value of VS2019.

This patch clears VSINSTALLDIR and VCToolsVersion env, then
the different vcvars32 can set the correct VS env.

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