Generate a CI build matrix automatically

[rw1nkler]

Currently, the CI's build matrix is created manually and contains all the valid combinations of different configuration variables. However, it is possible to generate the matrix automatically. An example of that can be found here: https://github.com/SymbiFlow/symbiflow-arch-pkgs/blob/master/.github/workflows/build.yml#L9-L32 This can potentially help us simplify the CI configuration, and the tuttest setup (see https://github.com/SymbiFlow/symbiflow-examples/pull/88#discussion_r541061734)

It is also worth to check other possible approaches to this problem.