Add nightly toolchain to test coverage CI.

.github/workflows/ci.yml

@@ -282,13 +282,20 @@ jobs:
             target
           key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
 
-      - name: Install toolchain
+      - name: Install stable toolchain
         uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
           override: true
           default: true
 
+      - name: Install nightly toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+          default: false
+
       - name: Install cargo-tarpaulin
         uses: actions-rs/install@v0.1
         with: