CI: Fix Nix workflows

- General Flake updates
- Move CI Nix setup to reusable action
- Replace deprecated `DeterminateSystems/magic-nix-cache-action@main`[1]
  with `nix-community/cache-nix-action@v6`

[1] https://determinate.systems/posts/magic-nix-cache-free-tier-eol/

Author: doshitan

.github/actions/setup-nix/action.yml

@@ -0,0 +1,12 @@
+name: "Setup Nix"
+description: "Install and configure Nix for Github Actions"
+runs:
+  using: "composite"
+  steps:
+    - uses: nixbuild/nix-quick-install-action@v30
+    - uses: nix-community/cache-nix-action@v6
+      with:
+        # restore and save a cache using this key
+        primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock', "uv.lock") }}
+        # if there's no cache hit, restore a cache by this prefix
+        restore-prefixes-first-match: nix-${{ runner.os }}-

.github/workflows/cd-docs.yml

@@ -21,8 +21,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: ./.github/actions/setup-nix
 
       - run: nix build '.#docs'
 

.github/workflows/ci-nix-template.yml

@@ -18,8 +18,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: ./.github/actions/setup-nix
 
       - name: Run build
         run: nix build
@@ -31,8 +30,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: ./.github/actions/setup-nix
       - uses: DeterminateSystems/flake-checker-action@main
 
       - name: Run lint
@@ -46,8 +44,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/configure-git
-      - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: ./.github/actions/setup-nix
 
       - name: Run tests
         run: nix develop --command bash -c "make test"
@@ -60,8 +57,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/configure-git
-      - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: ./.github/actions/setup-nix
 
       - name: Run install
         run: nix profile install '.'