allow setting vc_* properties in participants_matrix.cl:

```
participants_matrix:
  el:
    - el_type: nethermind
      el_image: nethermindeth/nethermind:pectra
    - el_type: geth
      el_image: ethpandaops/geth:lightclient-prague-devnet-1
  cl:
    - cl_type: teku
      cl_image: ethpandaops/teku:master
    - cl_type: nimbus
      cl_image: ethpandaops/nimbus-eth2:unstable
    - cl_type: lodestar
      cl_image: ethpandaops/lodestar:electra-fork
      vc_max_mem: 1024
    - cl_type: grandine
      cl_image: ethpandaops/grandine:electra
```

in the example above, lodestar vc needs more than 500M memory for key
import, so I'd like to override the vc_max_mem accordingly.
This is theoretically allowed by the logic, but the new sanity check
prevents it.