build: fix detection of Visual Studio 2019

When run in a Visual Studio 2019 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 2017.

vcbuild.bat

@@ -243,6 +243,10 @@ if %target_arch%==x86 if %msvs_host_arch%==x86 set vcvarsall_arg=x86
 :vs-set-2017
 if defined target_env if "%target_env%" NEQ "vs2017" goto vs-set-2019
 echo Looking for Visual Studio 2017
+@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
+if not defined target_env set "VCINSTALLDIR="
 call tools\msvs\vswhere_usability_wrapper.cmd "[15.0,16.0)"
 if "_%VCINSTALLDIR%_" == "__" goto msbuild-not-found
 if defined msi (