Upgrade CI actions to latest versions

- DeterminateSystems/nix-installer-action: v9 → v21
- DeterminateSystems/magic-nix-cache-action: v2 → v13
- cachix/cachix-action: v14 → v16

Fixes CI failures caused by TypeError: Invalid URL in the old
nix-installer-action when fetching from install.determinate.systems.

Author: angerman

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

@@ -3,14 +3,14 @@ description: Installs and configures nix and related tools
 runs:
   using: "composite"
   steps:
-    - uses: DeterminateSystems/nix-installer-action@v9
+    - uses: DeterminateSystems/nix-installer-action@v21
       with:
         github-token: ${{ env.GITHUB_TOKEN }}
         extra-conf: accept-flake-config = true
 
-    - uses: DeterminateSystems/magic-nix-cache-action@v2
+    - uses: DeterminateSystems/magic-nix-cache-action@v13
 
-    - uses: cachix/cachix-action@v14
+    - uses: cachix/cachix-action@v16
       with:
         name: foliage
         authToken: ${{ env.CACHIX_AUTH_TOKEN }}