simplify to assemble, not build

avoids the workflow doing a ton of API calls

Author: patrickthornton

.github/workflows/preview_sdks.yml

@@ -59,4 +59,4 @@ jobs:
           FERN_TOKEN: ${{ secrets.FERN_TOKEN }}
         run: |
           cd fern/.preview/fern-java-sdk
-          ./gradlew build
+          ./gradlew assemble