Drop magic-nix-cache-action — Determinate shut down the S3 bucket

The action returns 404 fetching its binary from
magic-nix-cache20230623203105255200000001.s3.dualstack.us-east-2.amazonaws.com,
breaking every CI and release run. Determinate's replacement is the paid
FlakeHub Cache. Remove the cache step entirely — builds still work,
just without a Nix store cache between runs.

Author: ananthb

.github/workflows/ci.yml

@@ -12,7 +12,6 @@ jobs:
       - uses: actions/checkout@v4
 
       - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
 
       - name: Build
         run: nix build

.github/workflows/release.yml

@@ -13,7 +13,6 @@ jobs:
       - uses: actions/checkout@v4
 
       - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
 
       - name: Build release tarball
         run: |
@@ -35,7 +34,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
 
       - name: Build package
         run: nix build .#packages.aarch64-darwin.default -o result