The code was a safe path to avoid sources access a not set property
This property belongs to the GitCache source itself. In fact, it was
overwritten there with a different value.

* Rename build_dir to be consistent with what sources handle,
config_src_dir.
* Initialize and set the config_src_dir in the Source
* Initialize and set the config_build_dir in the Build
* Use a new var, sources_dir to refer to the place where the build
system sources are located.
* Always override config_build_dir so the Oven can shell into
the correct path
* Allow building into a folder different than the config_src_dir
on Meson, CMake and Autotools

This will allow in-place projects without the need to actually
fetch the sources. Now projects can be delivered with a cerbero
recipe as part of the distfiles if required. Also, simplifies
testing by having local projects and recipes.

Ref #RDI-2209

Rust version, 1.84.0, will use emsdk 3.1.68 and this unblocks issues
with gst-plugins-rs

Issue: RDI_2506