FetchContent: Only use @-@ replacements for sub-build CMakeLists.txt

There was a mix of ${} and @-@ substitutions in the CMakeLists.txt.in
file. It is sometimes useful to temporarily put ${} evaluations there while
working on issues, but they get replaced. Using @ONLY leaves them
untouched and uses a consistent substitution method for the whole file.

Modules/FetchContent.cmake

@@ -1596,7 +1596,9 @@ set_property(GLOBAL PROPERTY _CMAKE_FindGit_GIT_EXECUTABLE_VERSION
   # has this set to something not findable on the PATH. We also ensured above
   # that the Debug config will be defined for multi-config generators.
   configure_file("${CMAKE_CURRENT_FUNCTION_LIST_DIR}/FetchContent/CMakeLists.cmake.in"
-                 "${ARG_SUBBUILD_DIR}/CMakeLists.txt")
+                 "${ARG_SUBBUILD_DIR}/CMakeLists.txt"
+                 @ONLY
+  )
   execute_process(
     COMMAND ${CMAKE_COMMAND} ${subCMakeOpts} .
     RESULT_VARIABLE result

Modules/FetchContent/CMakeLists.cmake.in

@@ -1,21 +1,21 @@
 # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
 # file Copyright.txt or https://cmake.org/licensing for details.
 
-cmake_minimum_required(VERSION ${CMAKE_VERSION})
+cmake_minimum_required(VERSION @CMAKE_VERSION@)
 
 # We name the project and the target for the ExternalProject_Add() call
 # to something that will highlight to the user what we are working on if
 # something goes wrong and an error message is produced.
 
-project(${contentName}-populate NONE)
+project(@contentName@-populate NONE)
 
 @__FETCHCONTENT_CACHED_INFO@
 
 include(ExternalProject)
-ExternalProject_Add(${contentName}-populate
-                    ${ARG_EXTRA}
-                    SOURCE_DIR          "${ARG_SOURCE_DIR}"
-                    BINARY_DIR          "${ARG_BINARY_DIR}"
+ExternalProject_Add(@contentName@-populate
+                    @ARG_EXTRA@
+                    SOURCE_DIR          "@ARG_SOURCE_DIR@"
+                    BINARY_DIR          "@ARG_BINARY_DIR@"
                     CONFIGURE_COMMAND   ""
                     BUILD_COMMAND       ""
                     INSTALL_COMMAND     ""