GitHub: don't cache the Rust target/ directory for the iOS tests

It's causing problems with the new-style iOS build.

.github/workflows/tests.yml

@@ -80,12 +80,6 @@ jobs:
       - name: Add rust-src component
         run: rustup component add rust-src
 
-      - name: Cache cargo build
-        uses: actions/cache@v1
-        with:
-          path: target
-          key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
-
       - name: Run pod lint
         # No import validation because it tries to build unsupported platforms (like 32-bit iOS).
         run: pod lib lint --verbose --platforms=ios --skip-import-validation --allow-warnings