win-dshow: Set current working directory in VirtualCam scripts

plugins/win-dshow/virtualcam-install.bat.in

@@ -1,4 +1,5 @@
 @echo off
+@cd /d "%~dp0"
 goto checkAdmin
 
 :checkAdmin

plugins/win-dshow/virtualcam-uninstall.bat.in

@@ -1,4 +1,5 @@
 @echo off
+@cd /d "%~dp0"
 goto checkAdmin
 
 :checkAdmin