Publish SNAPSHOT artifacts during PR builder

[thibauult]

Summary

• Adds a Publish SNAPSHOT artifacts step to the PR builder workflow
• The step only runs on pull_request events (skipped on pushes to main/*-rc)
• Uses publishToSonatype which automatically targets the Sonatype snapshot repository for -SNAPSHOT versions
• No GPG signing required — already gated on isReleaseVersion in bdk.java-publish-conventions.gradle

Prerequisites

MAVEN_USERNAME and MAVEN_PASSWORD repository secrets must be set (already used by release.yml).

Test plan

• Open a PR and verify the Publish SNAPSHOT artifacts step runs and succeeds
• Verify artifacts are published to https://ossrh-staging-api.central.sonatype.com/content/repositories/snapshots/
• Verify the step is skipped on a direct push to main