Fix nightly workflow build failure (install JDK/sbt, update artifact paths)

[vgarg-mf]

Summary

The Nightly Testing workflow's build job was failing almost immediately
(~3 seconds) with an sbt: command not found error. sbt (and the expected JDK)
are no longer preinstalled on GitHub's ubuntu-latest runner images, and
nightly.yml went straight from checkout to sbt package without setting them up.

Changes

• Add JDK 21 + sbt setup to the build job, mirroring the existing setup in
  main.yml, so sbt package can run.
• Fix the build artifact path from scala-2.12/spark-vertica-connector_2.12-*.jar
  to scala-2.13/spark-vertica-connector*.jar to match the current build output.
• Bump actions/download-artifact@v2 → @v4 in the downstream integration-test
  jobs, since v4 artifacts (produced by upload-artifact@v4) are not compatible with
  the v2 download action.

Impact

• Restores the nightly build job.
• Prevents cascading failures in run-spark-integration-tests and
  run-integration-tests-on-standalone-cluster (which depend on the build artifact).
• The Post Workflow Status To Slack job will no longer fire on this failure once the
  build succeeds.

Testing

• Can be validated by manually triggering the workflow via workflow_dispatch
  ("Run workflow") on the Actions tab, or on the next scheduled nightly run.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.92%. Comparing base (4d100bf) to head (0e030b4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #568   +/-   ##
=======================================
  Coverage   87.92%   87.92%           
=======================================
  Files          44       44           
  Lines        1963     1963           
  Branches      118      118           
=======================================
  Hits         1726     1726           
  Misses        237      237           

Flag	Coverage Δ
unittests	87.92% <ø> (ø)

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