add ci check for -default +unstable

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Author: Keruspe

.github/workflows/ci.yml

@@ -86,6 +86,12 @@ jobs:
         command:  check
         args: --features attributes
 
+    - name: build unstable only
+      uses: actions-rs/cargo@v1
+      with:
+        command:  build
+        args: --no-default-features --features unstable
+
     - name: tests
       uses: actions-rs/cargo@v1
       with: