appveyor: add executable name to the command

.appveyor.yml

@@ -16,7 +16,7 @@ init:
   - ps: iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
 
 install:
-  - '"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build" x64'
+  - '"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" x64'
   - choco install nsis
   - choco install opencv
   - 'C:\Tools\opencv\build\setup_vars_opencv4.cmd'