take into account location of header files of dependencies in toolchain method that defines search path for header files in build environment

[boegel]

fixes #5039, follow-up for #5023 which was a step in the right direction but incomplete

The _add_dependency_cpp_headers method of Toolchain only consider include as subdirectory for dependencies, but in some cases additional subdirectories must be taken into account for finder header files.
We can determine the correct list of subdirectories based on the $CPATH, and $C_INCLUDE_PATH & friends environment variables that get set when the modules for the (build) dependencies get loaded.

This fix is only relevant when EasyBuild is configured with --search-path-cpp-headers to something else than (the default) flags, since then $CPPFLAGS is defined in toolchain build environment and the $CPATH or $C_INCLUDE_PATH set by dependency modules is not overwritten).

[bedroge]

Lgtm!

Tested this with the examples from #5039 (comment) and various (module) search path configurations, and it works fine in all situations! 🎉

As an example, $CPPFLAGS is now correctly set to:

== 2025-11-12 20:05:56,552 environment.py:95 INFO Environment variable CPPFLAGS set to -I/data/apps/software/foo/1.0-GCC-13.3.0/include/foo -I/data/apps/software/foo/1.0-GCC-13.3.0/include (previously undefined)

And when using include_paths:

== 2025-11-13 11:06:38,118 environment.py:95 INFO Environment variable C_INCLUDE_PATH set to /data/apps/software/foo/1.0-GCC-13.3.0/include/foo:/data/apps/software/foo/1.0-GCC-13.3.0/i
nclude (previous value: '/data/apps/software/foo/1.0-GCC-13.3.0/include/foo:/data/apps/software/foo/1.0-GCC-13.3.0/include')

(it used to only include ..../include, and not ..../include/foo).