External Updates, main branch (2021.11.08.)

Updated the VecMem and Vc builds.

Vc is now downloaded as a TGZ file, instead of downloading the entire Git repository.
- This was bothering me quite a bit in recent weeks. That it currently takes a long time to download Vc for the build.
VecMem is updated to version v0.7.0, and the "language code" from it is activated for this project.
- In case we ever want to add any CUDA/HIP/SYCL tests to this project.
- This is done in the same way as in https://github.com/acts-project/traccc/pull/106.

I also enabled the setup of VecMem by default for the project. Every other "traccc project" depends on VecMem by now. We may want to make that mandatory here as well. Though maybe not... I could possibly be convinced of the contrary... :thinking:

acts-project / algebra-plugins

External Updates, main branch (2021.11.08.) #38