ci: drop actions/cache for cargo registry and target

The cache round-trip (download + unpack) was consistently taking
around 6 minutes, noticeably longer than the ~3 minute cold build
it was meant to accelerate. Net-negative on CI time — remove it.

sccache with the S3 backend still provides dep-level caching at a
much lower overhead, so we keep the majority of the cache benefit
without paying the actions/cache tarball cost.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -24,19 +24,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Cache cargo registry and target
-        uses: actions/cache@v4
-        with:
-          path: |
-            ~/.cargo/bin
-            ~/.cargo/registry/index
-            ~/.cargo/registry/cache
-            ~/.cargo/git/db
-            target
-          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-
-
       - name: Ensure sccache with S3 support
         env:
           RUSTC_WRAPPER: ""