feat(ci): add dedicated Nix cache prebuild runner

.github/workflows/build_systems.yml

@@ -8,8 +8,23 @@ on:
     - cron: "0 22 * * *"
 
 jobs:
+  prebuild-common:
+    name: prebuild-common-x86-64-v3
+    runs-on: nix-cache-builder
+    steps:
+      - uses: actions/checkout@v4
+      # portal-1 is the smallest system closure: 95% of its derivations are
+      # shared by all five systems, so it is a maintainable common cache seed.
+      # Keep going so one failing package does not stop unrelated cache entries
+      # from being built.
+      - name: Build common packages
+        run: nixos-rebuild build --keep-going --accept-flake-config --flake ./#portal-1
+      - name: Copy common packages to nix-cache
+        run: nix copy --accept-flake-config --to unix:///host-nix/var/nix/daemon-socket/socket .#nixosConfigurations.portal-1.config.system.build.toplevel
+
   build:
     name: build-${{ matrix.system }}
+    needs: prebuild-common
     runs-on: self-hosted
     strategy:
       matrix: