build: conditionally clear vcinstalldir

For scenario where target env is explicitly specified as vs2019, do
not clear VCINSTALLDIR which was being cleared to handle fallback to
vs2017 block when attempting to find a matching available VS.

Fixes: #35856

PR-URL: #36009
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

vcbuild.bat

@@ -249,7 +249,7 @@ echo Looking for Visual Studio 2019
 @rem VCINSTALLDIR may be set if run from a VS Command Prompt and needs to be
 @rem cleared first as vswhere_usability_wrapper.cmd doesn't when it fails to
 @rem detect the version searched for
-set "VCINSTALLDIR="
+if not defined target_env set "VCINSTALLDIR="
 call tools\msvs\vswhere_usability_wrapper.cmd "[16.0,17.0)"
 if "_%VCINSTALLDIR%_" == "__" goto msbuild-not-found
 set "WIXSDKDIR=%WIX%\SDK\VS2017"