ci: use Gradle wrapper for release builds

[sgallese]

Summary

• Use the repository Gradle wrapper in release build workflows.
• Avoid failures when GitHub runners provide an incompatible ambient Gradle version.

Test plan

• Reproduced the current build failure locally with Gradle 9.6.1: gradle assembleGithubRelease fails during Android plugin configuration.
• Verified the fix locally: ./gradlew assembleGithubRelease completes successfully with Gradle 8.13.

[Ashinch]

Thanks for the fix! This was breaking every pull request build. Appreciate the help.