Use VS from env if specified (#25749)

startvs.cmd

@@ -27,4 +27,8 @@ IF NOT EXIST "%DOTNET_ROOT%\dotnet.exe" (
     exit /b 1
 )
 
-start "" "%sln%"
+IF "%VSINSTALLDIR%" == "" (
+    start "" "%sln%"
+) else (
+    "%VSINSTALLDIR%\Common7\IDE\devenv.com" "%sln%"
+)