[ci] Have an OSS-Formal CI Pass

This commit introduces a new ci job which invokes the OS formal
flow. It uses the nix flake, which contains all the necessary
dependencies.

With everything already in cache this takes less than an hour on pomona.

Author: mndstrmr

.github/workflows/ci-formal.yml

@@ -0,0 +1,57 @@
+# Copyright lowRISC contributors.
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
+# GitHub Actions CI build configuration
+
+name: Ibex OSS Formal CI
+
+on:
+  push:
+    paths:
+      - 'flake.nix'
+      - 'flake.lock'
+      - 'dv/formal/**'
+      - 'rtl/**'
+      - 'vendor/**'
+      - '.github/**'
+  merge_group:
+    types:
+      - checks_requested
+  pull_request:
+    paths:
+      - 'flake.nix'
+      - 'flake.lock'
+      - 'dv/formal/**'
+      - 'rtl/**'
+      - 'vendor/**'
+      - '.github/**'
+
+jobs:
+  fv:
+    name: Run the Open-Source FV flow
+    runs-on: nixos
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup env
+        run: |
+          echo "NIX_CONFIG=accept-flake-config = true" >> $GITHUB_ENV
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v27
+        with:
+          extra_nix_config: |
+            substituters = https://nix-cache.lowrisc.org/public/ https://cache.nixos.org/
+            trusted-public-keys = nix-cache.lowrisc.org-public-1:O6JLD0yXzaJDPiQW1meVu32JIDViuaPtGDfjlOopU7o= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+
+      - name: Run OSS Env
+        run: |
+          source <(nix print-dev-env .#oss-dev)
+
+          cd dv/formal
+          make build/aig-manip
+          make build/all.aig SHELL=bash
+          python3 conductor.py prove --check-complete
+          # add --max-mem <gb> to restrict to some upper bound on memory usage. Hard minimum is 20 but 128 is a more realistic minimum
+