test: ensure java is on PATH in integration container

[Technoboy-]

Summary

• add a small PATH/JAVA_HOME fallback in the integration test entrypoint
• look for java in the JDK locations used by the 4.3.0-SNAPSHOT snstage image
• keep this PR scoped to the JDK fix only for validation

Testing

• validated that /opt/jvm/bin/java exists in snstage/pulsar-all:4.3.0-SNAPSHOT
• validated the entrypoint script with bash -n
• observed sink/source integration failures move past the previous "java not found" error after this fix

[github-actions]

@Technoboy-:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)