Previously, the build directory was not explicitly specified. Instead, the
current working directory was assumed to be the build directory and the
source directory was passed as a positional argument. However, if the
source directory contained a CMakeCache.txt, it was treated as a binary
directory instead. For the scenario where a project is built in-source and
it calls ExternalProject_Add() with its own source directory but a different
build directory (e.g. to build itself with a different set of configure arguments),
this results in a build error.

Explicitly list both the source and build directories using the -S and -B options
respectively. This unambiguously sets both, avoiding the above problem.

Fixes: #24081