CI: Build benchmarks
fixes: #599 (closed)
It adds benchmark to the builds:
- Ubuntu 18.04 GCC7 + CUDA
- Ubuntu 18.04 GCC9 + TDD
- Ubuntu 18.04 Kokkos
In each instantiation Google Benchmark is downloaded and built, this is a design decision to let us change its version easily without breaking the docker image.
Note that we cache the object files of Google Benchmark with sccache, thus the overhead of downloading/compiling Gbench is minimal