FetchContent: Use Debug rather than Release for sub-build

The config used can be part of the path to the sub-build. On some
platforms, there are restrictions on the length of paths or file names.
Use Debug rather than Release because the former is shorter.
When CMAKE_CONFIGURATION_TYPES is left at its default, this also
makes the path the same as the previous default before 10865c8e5f
(FetchContent: Hard-code a config for the sub-build, 2022-05-01) .