👷 Add install-lix action and build workflow

.github/actions/install-lix/action.yml

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+# Taken from: https://github.com/isabelroses/dotfiles/blob/9335e5728eaaa9bdcb0d5c3130a5d7c8dddbb6e3/.github/actions/install-lix/action.yml :3
+name: Install Lix
+description: "Install Lix for the specified system"
+
+runs:
+  using: composite
+
+  steps:
+    - uses: DeterminateSystems/nix-installer-action@main
+      with:
+        source-url: 'https://install.lix.systems/lix/lix-installer-${{ fromJSON(''{"X64":"x86_64","X86":"i686","ARM64":"aarch64","ARM":"armv7l"}'')[runner.arch] }}-${{ fromJSON(''{"Linux":"linux","macOS":"darwin","Windows":"windows"}'')[runner.os] }}'
+        logger: pretty
+        diagnostic-endpoint: ""
+        extra-conf: |
+          experimental-features = flakes nix-command
+          substituters = https://nix-community.cachix.org  https://catppuccin.cachix.org https://cache.nixos.org/
+          trusted-public-keys = nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs= catppuccin.cachix.org-1:noG/4HkbhJb+lUAdKrph6LaozJvAeEEZj4N732IysmU= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=

.github/workflows/build.yml

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+name: "Nix: build and cache outputs"
+
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - "**.nix"
+      - "**.lock"
+      - ".github/workflows/build.yml"
+
+jobs:
+  build-flake:
+    name: Build (x86_64-linux)
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.ref_name }}
+          persist-credentials: false
+
+      - name: Install Lix
+        uses: ./.github/actions/install-lix
+
+      - name: Setup Attic cache
+        uses: ryanccn/attic-action@v0.3.2
+        with:
+          endpoint: https://cache.thevoid.cafe
+          cache: puzzlevision
+          token: ${{ secrets.ATTIC_TOKEN }}
+
+      - name: Build flake
+        run: nix build