NSIS: Elevate uninstaller, too

Resolves #3546

cmake/modules/NSIS.template.in

@@ -789,6 +789,7 @@ Function un.onInit
    Call un.SetLang
 
    UAC_TryAgain:
+      !insertmacro UAC_RunElevated
       ${Switch} $0
       ${Case} 0
           ${IfThen} $1 = 1 ${|} Quit ${|} ;we are the outer process, the inner process has done its work, we are done