ci: build and test with and without unstable features

Author: phip1611

.github/workflows/rust.yml

@@ -44,9 +44,14 @@ jobs:
         sudo apt-get update
         sudo apt-get install qemu-system-x86 ovmf -y
 
-    - name: Build
+    - name: Build (without unstable)
       run: cargo xtask build --target x86_64
 
+      # It is sufficient if we include the "--include-unstable" flag only for one build as we
+      # only plan to have platform-independent functionality behind basic feature gates.
+    - name: Build (with unstable)
+      run: cargo xtask build --target x86_64 --include-unstable
+
     - name: Run VM tests
       run: cargo xtask run --target x86_64 --headless --ci
       timeout-minutes: 2
@@ -77,9 +82,12 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v2
 
-      - name: Run cargo test
+      - name: Run cargo test (without unstable)
         run: cargo xtask test
 
+      - name: Run cargo test (with unstable)
+        run: cargo xtask test --include-unstable
+
   lints:
     name: Lints
     runs-on: ubuntu-latest