Fix build with --features=v2021_3, use in CI by default

It's a huge trap for us not to build with the latest ostree feature
on, I didn't have my IDE configured for it, and CI didn't have
it on.

The previous bump to glib 0.14 broke the Sign code.

rust-bindings/rust/.github/workflows/rust.yml

@@ -18,6 +18,6 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Build
-      run: cargo build --verbose
+      run: cargo build --verbose --features=v2021_3
     - name: Run tests
-      run: cargo test --verbose
+      run: cargo test --verbose --features=v2021_3