use test cache for build lib

.github/workflows/ci.yml

@@ -180,6 +180,8 @@ jobs:
     needs: [registry-cache, check]
     runs-on: ubuntu-20.04
     timeout-minutes: 20
+    outputs:
+      cache-key: ${{ steps.cache-key.outputs.key }}
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
@@ -201,11 +203,16 @@ jobs:
             ~/.cargo/git
           key: ${{ needs.registry-cache.outputs.cache-key }}
 
+      - name: Generate build artifacts key
+        id: cache-key
+        shell: bash
+        run: echo "::set-output name=key::$(echo "${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-tests-${{ needs.registry-cache.outputs.cache-date }}-${{ hashFiles('**/Cargo.lock') }}")"
+
       - name: Cache build artifacts
         uses: actions/cache@v2
         with:
           path: ${{ github.workspace }}/target
-          key: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-tests-${{ needs.registry-cache.outputs.cache-date }}-${{ hashFiles('**/Cargo.lock') }}
+          key: ${{ steps.cache-key.outputs.key }}
           restore-keys: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-tests-${{ needs.registry-cache.outputs.cache-date }}-
 
       - name: Download data
@@ -476,10 +483,14 @@ jobs:
         uses: actions/cache@v2.1.4
         with:
           path: ${{ github.workspace }}/target
-          key: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-build-linux-${{ needs.registry-cache.outputs.cache-date }}-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-build-linux-${{ needs.registry-cache.outputs.cache-date }}-
+          key: ${{ needs.test.outputs.cache-key }}
 
       - name: Build linux lib
+        # We have to use the same RUSTFLAGS that are used in the test
+        # job in order to be able to reuse the cache. If we do not do this,
+        # the compiler will recompile all the libraries from scratch.
+        env:
+          RUSTFLAGS: "-D warnings"
         run: cargo build
 
       - name: Generate lib artifacts key