ci: prebuild the example Android project before the Gradle test

The example is an Expo CNG app — its android/ project is generated, not committed
(example/my-app/.gitignore), so the Gradle step failed in CI with "working directory
.../example/my-app/android: No such file or directory". Add an
`npx expo prebuild --platform android --no-install` step so the android/ project (and
the autolinked :react-native-nitro-healthkit subproject) exists before ./gradlew runs.

Verified locally that prebuild generates android/gradlew and expo-modules-autolinking
resolves the module with packageImportPath
`import com.margelo.nitro.healthkit.NitroHealthkitPackage;`.

Author: N0ku

.github/workflows/ci.yml

@@ -71,6 +71,15 @@ jobs:
         working-directory: example/my-app
         run: npm ci
 
+      # The example is an Expo CNG app — its android/ project is generated, not
+      # committed (see example/my-app/.gitignore). Generate it so the library is
+      # autolinked as the `:react-native-nitro-healthkit` Gradle subproject.
+      - name: Prebuild example Android project
+        working-directory: example/my-app
+        run: npx expo prebuild --platform android --no-install
+        env:
+          EXPO_NO_TELEMETRY: "1"
+
       - name: Gradle :react-native-nitro-healthkit:test
         working-directory: example/my-app/android
         run: ./gradlew :react-native-nitro-healthkit:test --no-daemon