ci: remove build-cache job as lockfile hash handles invalidation

Now that cache keys include lockfile hashes, proper invalidation
happens automatically. Each job can build and save its own cache,
making the dedicated build-cache job unnecessary.

Author: ryoppippi

.github/workflows/ci.yaml

@@ -16,25 +16,7 @@ permissions:
   id-token: write
 
 jobs:
-  build-cache:
-    runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        python-version: ["python311", "python313"]
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-
-      - name: Setup Nix
-        uses: ./.github/actions/setup-nix
-        with:
-          python-version: ${{ matrix.python-version }}
-
-      - name: Build Nix environment
-        run: nix develop .#${{ matrix.python-version }} --command true
-
   gitleaks:
-    needs: build-cache
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
@@ -49,7 +31,6 @@ jobs:
         run: nix develop --command just gitleaks
 
   ci:
-    needs: build-cache
     runs-on: ubuntu-latest
     strategy:
       matrix:
@@ -75,7 +56,6 @@ jobs:
         run: nix develop .#${{ matrix.python-version }} --command just test
 
   coverage:
-    needs: build-cache
     runs-on: ubuntu-latest
     if: github.ref == 'refs/heads/main'
     steps: