ExternalProject: Express per-config step stamp file paths using CONFIG genex

The ExternalProject module has long used the generator-specific
placeholder in the `${CMAKE_CFG_INTDIR}` variable to express per-config
stamp file paths in multi-config generators.  Now that most generators
support generator expressions in custom command outputs, we can use
the `$<CONFIG>` genex instead.

In particular, this fixes cross-config `BUILD_BYPRODUCTS` with the Ninja
Multi-Config generator.

Fixes: #23595