ExternalProject: Fix regression in stamp creation for Xcode+iOS

The change to `_ep_add_mkdir_command` in commit 5fbac2bb
(ExternalProject: Move inline scripts to separate files, 2022-01-22,
v3.23.0-rc1~101^2) did not account for the possibility that
`CMAKE_CFG_INTDIR` is `$(CONFIGURATION)$(EFFECTIVE_PLATFORM_NAME)`
instead of just the configuration name.  Pass the value into the helper
script on the command line so that the native buildsystem placeholders
are evaluated.

Fixes: #23645