ci: remove Magic Nix Cache

Due to the large number of derivations we are building, this almost
always gets disabled due to rate limits before it's able to fetch
anything which isn't already on cache.nixos.org. Rate limiting also
spams the log with warnings which are unrelated to the build.

.github/workflows/check.yml

@@ -13,7 +13,6 @@ jobs:
 
     steps:
       - uses: DeterminateSystems/nix-installer-action@v16
-      - uses: DeterminateSystems/magic-nix-cache-action@v8
 
       - id: get-derivations
         run: |
@@ -66,7 +65,6 @@ jobs:
 
     steps:
       - uses: DeterminateSystems/nix-installer-action@v16
-      - uses: DeterminateSystems/magic-nix-cache-action@v8
 
       - run: |
           nix build --no-update-lock-file --print-build-logs \

.github/workflows/docs.yml

@@ -26,7 +26,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@v16
-      - uses: DeterminateSystems/magic-nix-cache-action@v8
       - run: nix build .#docs
 
       - uses: actions/upload-pages-artifact@v3