Configure pgJDBC root certificate in DSQL integration test

[spencercorwin]

Summary

• provision the Ubuntu trusted CA bundle at pgJDBC's default root.crt path
• retain sslmode=verify-full certificate and hostname verification
• unblocks CI runs for other PRs

Root cause

The Aurora DSQL sample URL was changed from sslmode=require to
sslmode=verify-full, but the GitHub Actions runner did not provide
${user.home}/.postgresql/root.crt. pgJDBC therefore failed before
authentication or benchmark execution.

Verification

• git diff --check
• parsed .github/workflows/dsqlmaven.yml with Ruby YAML
• verified that certificate setup allowed the DSQL integration workload to
  create, load, and execute the TPC-C benchmark
• fresh end-to-end DSQL integration rerun passed