ci: Fix test environment covering CMAKE_CXX_STDLIB_MODULES_JSON

In commit 85e536b5eb (gitlab-ci: test `CMAKE_CXX_STDLIB_MODULES_JSON` in
CI, 2025-09-11, v4.2.0-rc1~182^2) we intended to move the file
`libstdc++.modules.json` to a new folder, but source and destination
paths for the `file(RENAME ...)` command were the same. They both were
the original location of the file.  Fix the code to move the file to the
directory created with `file(MAKE_DIRECTORY ...)`.

Fixes: #27586

.gitlab/ci/configure_linux_gcc_cxx_modules_reloc_ninja.cmake

@@ -2,7 +2,7 @@
 # `CMAKE_CXX_STDLIB_MODULES_JSON` is needed to use `import std`.
 set(gcc_prefix "/opt/gcc-importstd")
 set(CMake_TEST_CXX_STDLIB_MODULES_JSON
-  "${gcc_prefix}/lib64/libstdc++.modules.json"
+  "${gcc_prefix}/lib64.reloc/libstdc++.modules.json"
   CACHE FILEPATH "")
 file(MAKE_DIRECTORY
   "${gcc_prefix}/lib64.reloc")