Fix CI caching for bdk-ffi/target

.github/workflows/test.yaml

@@ -20,7 +20,7 @@ jobs:
           path: |
             ~/.cargo/registry
             ~/.cargo/git
-            target
+            bdk-ffi/target
           key: ${{ runner.os }}-${{ hashFiles('**/Cargo.toml','**/Cargo.lock') }}
 
       - name: Set up JDK