build: fix detection of Visual Studio 2017

When run in a Visual Studio 2017 command prompt the `VCINSTALLDIR`
environment variable will be already set and is not cleared by the
`tools/msvs/vswhere_usability_wrapper.cmd` utility when it fails to
find Visual Studio 2019. This causes `vcbuild.bat` to incorrectly
assume Visual Studio 2019 and generate an incompatible configuration.

Clearing the value of `VCINSTALLDIR` before calling the utility fixes
the detection logic.

PR-URL: #30119
Fixes: #30118
Refs: #30022
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>

vcbuild.bat

@@ -240,6 +240,10 @@ if %target_arch%==x86 if %msvs_host_arch%==x86 set vcvarsall_arg=x86
 :vs-set-2019
 if defined target_env if "%target_env%" NEQ "vs2019" goto vs-set-2017
 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="
 call tools\msvs\vswhere_usability_wrapper.cmd "[16.0,17.0)"
 if "_%VCINSTALLDIR%_" == "__" goto vs-set-2017
 if defined msi (