NSIS: Fix incorrect uninstall registry key name (#13578)

Thanks to Alexander for the bug report.

Modules/NSIS.template.in

@@ -902,7 +902,7 @@ SectionEnd
 Function .onInit
   StrCmp "@CPACK_NSIS_ENABLE_UNINSTALL_BEFORE_INSTALL@" "ON" 0 inst
 
-  ReadRegStr $0 HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\@CPACK_PACKAGE_NAME@" "UninstallString"
+  ReadRegStr $0 HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\@CPACK_PACKAGE_INSTALL_REGISTRY_KEY@" "UninstallString"
   StrCmp $0 "" inst
 
   MessageBox MB_YESNOCANCEL|MB_ICONEXCLAMATION \