Added missing ci/appveyor/install_grip.bat

Author: end2endzone

ci/appveyor/install_grip.bat

@@ -0,0 +1,11 @@
+@echo off
+
+:: Validate appveyor's environment
+if "%APPVEYOR_BUILD_FOLDER%"=="" (
+  echo Please define 'APPVEYOR_BUILD_FOLDER' environment variable.
+  exit /B 1
+)
+
+:: Call matching script for windows
+call "%APPVEYOR_BUILD_FOLDER%\ci\windows\%~n0.bat"
+if %errorlevel% neq 0 exit /b %errorlevel%