ci: remove actions/cache step (gitea.com clone hangs 12+ min)

The actions/cache@v4 clone from gitea.com has been hanging for 12+
minutes in run 17. The actions/checkout clone took 2.5 min (slow but
completed), but actions/cache is stuck indefinitely.

Removing the cache step entirely. Trade-off: CI recompiles from scratch
each run (slower), but actually progresses past the action-clone phase.
Can re-add once gitea.com access is faster or actions are pre-cached
on the runner.

.github/workflows/ci.yml

@@ -84,17 +84,6 @@ jobs:
           registry = "sparse+https://rsproxy.cn/index/"
           EOF
 
-      - name: Cache cargo registry + build artifacts
-        uses: https://gitea.com/actions/cache@v4
-        with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: ${{ runner.os }}-cargo-${{ hashFiles('Cargo.lock', 'Cargo.toml') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-
-
       - name: Clippy (release, all targets)
         run: cargo clippy --release --all-targets