Add magic nix cache (again)

Should speed things up a tiny bit, particularly for the PR builds which
can't use our cache (yet)

.github/workflows/colmena.yml

@@ -39,6 +39,13 @@ jobs:
           extra_nix_config: |
             accept-flake-config = true
 
+        # Why use this when we have our own? Github's cache in this case is likely faster than ours, where it's possible to use. Larger paths generally aren't, so they'll fall back onto our cache
+      - name: Init Nix Magic Cache
+        uses: DeterminateSystems/magic-nix-cache-action@main
+        with:
+          diagnostic-endpoint: ""
+          use-flakehub: false
+
       - name: Setup attic Binary Cache
         if: github.repository == 'tgstation-operations/infrastructure' && github.ref == 'refs/heads/main'
         # Format for pointing to caches is server:cache in these commands
@@ -102,6 +109,12 @@ jobs:
             extra-substituters = https://attic.tgstation13.org/tgstation-infrastructure
             extra-trusted-public-keys = tgstation-infrastructure:tNpjd5GxK1xymRHsJdBLTpeDScA2mVPdKA/eIOLOE0I=
 
+      - name: Init Nix Magic Cache
+        uses: DeterminateSystems/magic-nix-cache-action@main
+        with:
+          diagnostic-endpoint: ""
+          use-flakehub: false
+
       - name: Authenticate Attic Binary Cache
         run: |
           nix profile install nixpkgs#attic-client