no need to cache target because it become too large (#37)

.github/workflows/test.yml

@@ -17,17 +17,12 @@ jobs:
       uses: actions/cache@v1
       with:
         path: ~/.cargo/registry
-        key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.toml') }}
+        key: ${{ runner.os }}-cargo-registry
     - name: Cache cargo index
       uses: actions/cache@v1
       with:
         path: ~/.cargo/git
-        key: ${{ runner.os }}-cargo-index-${{ hashFiles('**/Cargo.toml') }}
-    - name: Cache cargo build
-      uses: actions/cache@v1
-      with:
-        path: target
-        key: ${{ runner.os }}-cargo-build-target-${{ hashFiles('**/Cargo.toml') }}
+        key: ${{ runner.os }}-cargo-index
     - name: Install toolchain
       uses: actions-rs/toolchain@v1
       with: