actions: use nix-fast-build

.github/workflows/build-then-cache.yaml

@@ -17,5 +17,5 @@ jobs:
           cache: ${{ secrets.ATTIC_CACHE }}
           token: ${{ secrets.ATTIC_TOKEN }}
       # free useless disk space
-      - run: 'curl https://raw.githubusercontent.com/apache/arrow/31747f067a6ae29eca2d9bc68748500a8163d83e/ci/scripts/util_free_space.sh | bash'
+      - run: 'bash ci/util_free_space_extreme.sh'
       - run: 'nix build .#nixosConfigurations.nullbox.config.system.build.toplevel'

.github/workflows/proactive-update.yaml

@@ -20,9 +20,9 @@ jobs:
           cache: ${{ secrets.ATTIC_CACHE }}
           token: ${{ secrets.ATTIC_TOKEN }}
       # free useless disk space
-      - run: 'curl https://raw.githubusercontent.com/apache/arrow/31747f067a6ae29eca2d9bc68748500a8163d83e/ci/scripts/util_free_space.sh | bash'
+      - run: 'bash ci/util_free_space_extreme.sh'
       - run: 'nix flake update'
-      - run: 'nix build .#nixosConfigurations.rpi4.config.system.build.toplevel'
+      - run: 'nix develop .#ci --command nix-fast-build --no-nom --skip-cache -f .#nixosConfigurations.rpi4.config.system.build.toplevel'
 
   build_x86:
     runs-on: ubuntu-24.04
@@ -36,8 +36,8 @@ jobs:
           cache: ${{ secrets.ATTIC_CACHE }}
           token: ${{ secrets.ATTIC_TOKEN }}
       # free useless disk space
-      - run: 'curl https://raw.githubusercontent.com/apache/arrow/31747f067a6ae29eca2d9bc68748500a8163d83e/ci/scripts/util_free_space.sh | bash'
+      - run: 'bash ci/util_free_space_extreme.sh'
       - run: 'nix flake update'
-      - run: 'nix build .#nixosConfigurations.nullbox.config.system.build.toplevel'
+      - run: 'nix develop .#ci --command nix-fast-build --no-nom --skip-cache -f .#nixosConfigurations.nullbox.config.system.build.toplevel'
-      - run: 'nix build .#nixosConfigurations.slab.config.system.build.toplevel'
+      - run: 'nix develop .#ci --command nix-fast-build --no-nom --skip-cache -f .#nixosConfigurations.slab.config.system.build.toplevel'
-      - run: 'nix build .#nixosConfigurations.nixos-wsl.config.system.build.toplevel'
+      - run: 'nix develop .#ci --command nix-fast-build --no-nom --skip-cache -f .#nixosConfigurations.nixos-wsl.config.system.build.toplevel'

flake.nix

@@ -343,6 +343,12 @@
     devShells = eachSystem (system: let
       pkgs = import nixpkgs-unstable { inherit system; };
     in {
+      ci = pkgs.mkShell {
+        buildInputs = with pkgs; [
+          nix-update
+          nix-fast-build
+        ];
+      };
       default = pkgs.mkShell {
         buildInputs = with pkgs; [
           nix-update