Use mold linker in nix env on Linux (#2628)

* nix: use mold on linux

The build and test CI workflows already use mold. So this change only
affects development in the nix environments on Linux systems.

* CI: ensure nix dev env works on ARM based macOS

* chore: fix flake.nix on MacOS

* don't use pkgs in overlay

I guess when the overlay is applied there is a `pkgs` in scope, but not
sure. Probably more idiomatic to refer to attributes of `prev` which is
the package set before applying the overlay.

* CI: use `just build` instead of custom command

Also covers the sequencer-sqlite crate.

Author: sveitser

.github/workflows/nix-env-macos-arm.yml

@@ -0,0 +1,58 @@
+# This workflow ensures that the nix dev env works on ARM based macOS.
+name: Nix env macOS ARM
+
+on:
+  push:
+    branches:
+      - main
+      - release-*
+    tags:
+      # YYYYMMDD
+      - "20[0-9][0-9][0-1][0-9][0-3][0-9]*"
+  pull_request:
+    branches:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  RUST_LOG: info,libp2p=off
+
+jobs:
+  nix-env-macos-arm:
+    timeout-minutes: 60
+    runs-on: macos-latest
+    steps:
+      - name: Install Nix
+        uses: cachix/install-nix-action@v30
+
+      - name: Enable Cachix
+        uses: cachix/cachix-action@v15
+        # If PR is from a non-collaborator (e. g. dependabot) the secrets are missing and the login to cachix fails.
+        continue-on-error: true
+        with:
+          name: espresso-systems-private
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+          extraPullNames: nix-community
+          skipPush: ${{ github.actor == 'dependabot[bot]' }}
+
+      - name: Checkout Repository
+        uses: actions/checkout@v4
+        with:
+          submodules: recursive
+
+      - uses: Swatinem/rust-cache@v2
+        name: Enable Rust Caching
+        with:
+          prefix-key: v0-rust
+
+      - name: Activate nix env
+        run: nix develop --accept-flake-config -c echo Ok
+
+      - name: Build rust crates
+        run: nix develop --accept-flake-config -c just build
+
+      - name: Build contracts
+        run: nix develop --accept-flake-config -c forge build

flake.nix

@@ -66,6 +66,14 @@
           solhint =
             solhintPkg { inherit (prev) buildNpmPackage fetchFromGitHub; };
         })
+
+        # The mold linker is around 50% faster on Linux than the default linker.
+        # This overlays a mkShell that is configured to use mold on Linux.
+        (final: prev: prev.lib.optionalAttrs prev.stdenv.isLinux {
+          mkShell = prev.mkShell.override {
+            stdenv = prev.stdenvAdapters.useMoldLinker prev.clangStdenv;
+          };
+        })
       ];
       pkgs = import nixpkgs { inherit system overlays; };
       crossShell = { config }: