Verilator fetched from cache instead of rebuilt

[marnovandermaas]

This PR moves over to using Nix instead of building Verilator from scratch.

Fixes: #11

[elliotb-lowrisc]

I see in the GitHub log that it didn't seem to actually accept the lowRISC cache, nor did it seem to spend the time doing much of anything

Run nix build .#sunburst_simulator -L
warning: ignoring untrusted flake configuration setting 'extra-substituters'.
Pass '--accept-flake-config' to trust it
warning: ignoring untrusted flake configuration setting 'extra-trusted-public-keys'.
Pass '--accept-flake-config' to trust it

Is it actually working?

[marnovandermaas]

Thanks @elliotb-lowrisc for having a look at this. The Nix cache is added separately so it's fine to ignore the untrusted flake configuration. We should leave that in for the development environment though.

The reason why the building of the Simulator passes instantly is because it uses the Nix store on our CI runner. The build was done in this run: https://github.com/lowRISC/sunburst-chip/actions/runs/14176372643/job/39712156986

If we change something in the source code, this will trigger a rebuild.

[elliotb-lowrisc]

The development environment? Is there a way to use this locally as well then? Might be worth adding something to the README then

[marnovandermaas]

The development environment isn't super useful yet. I'll add instructions for that once I have the CHERIoT LLVM in there. I have put some documentation on how to build the simulator through Nix though.

[elliotb-lowrisc]

Thanks