add new toolchains with MPI disabled
For building my dune-dpg code I need to disable MPI; otherwise some of the tests won't successfully run. With Ansgar's old CI runners I simply could override the dune.opts to disable MPI but this is not possible with the new CI runners anymore.
The easiest way for me to fix it would be to add new toolchains to the CI runners which have MPI disabled. Those toolchains might also be useful for other projects as the non-MPI configuration is currently not tested during CI.