CPack/NSIS: Fix PATH modification without "bin" directory

Previously our NSIS project template hard-coded `$INSTDIR\bin` for
inclusion in the `PATH`.  If the directory does not exist, nothing is
done.  Use `CPACK_NSIS_EXECUTABLES_DIRECTORY` instead of `bin`.

Fixes: #15635

Modules/Internal/CPack/NSIS.template.in

@@ -742,7 +742,7 @@ Section "-Core installation"
 SectionEnd
 
 Section "-Add to path"
-  Push $INSTDIR\bin
+  Push $INSTDIR\@CPACK_NSIS_EXECUTABLES_DIRECTORY@
   StrCmp "@CPACK_NSIS_MODIFY_PATH@" "ON" 0 doNotAddToPath
   StrCmp $DO_NOT_ADD_TO_PATH "1" doNotAddToPath 0
     Call AddToPath
@@ -902,7 +902,7 @@ Section "Uninstall"
 
   DeleteRegKey /ifempty SHCTX "Software\@CPACK_PACKAGE_VENDOR@\@CPACK_PACKAGE_INSTALL_REGISTRY_KEY@"
 
-  Push $INSTDIR\bin
+  Push $INSTDIR\@CPACK_NSIS_EXECUTABLES_DIRECTORY@
   StrCmp $DO_NOT_ADD_TO_PATH_ "1" doNotRemoveFromPath 0
     Call un.RemoveFromPath
   doNotRemoveFromPath: