ci: remove Magic Nix Cache (#745)

Magic Nix Cache will no longer work with the GitHub Actions cache
as of February 1st [1].

Temporarily removing this while we look into alternative solutions.

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

.github/workflows/check.yml

@@ -17,7 +17,6 @@ jobs:
 
     steps:
       - uses: DeterminateSystems/nix-installer-action@v16
-      - uses: DeterminateSystems/magic-nix-cache-action@v8
 
       - id: get-derivations
         run: |
@@ -70,7 +69,6 @@ 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