verilog: unstable-2019-08-01 -> unstable-2020-08-24

[cole-h]

The build would previously fail due to a change in bison's behavior (see
steveicarus/iverilog@5b699c1
for more information). Updating to a more recent version fixes this
issue.

Motivation for this change

ZHF: #97479

Saw this was failing when looking for packages to fix for ZHF, and realized I need it for one of my classes this semester... Anyways, I tested this by compiling the first assignment (an 8-to-3 decoder) and verifying the output was the same. Test source can be found here: https://gist.github.com/cole-h/d499079a8ea0c58a80f415e9b5e2bafa.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

I'll backport this to 20.09 in the morning.

@NixOS/nixos-release-managers

[david-sawatzke]

Also fixes #97110

[david-sawatzke]

Yosys works with this iverilog version, compiled some litex with it, without issues