fix: configure Git user identity before Nebula version bump (#470)

[sakshar2303]

Summary

Fixes #470

Root Cause

The ./gradlew final (Nebula release plugin) creates a Git commit and
tag to bump the version. The GitHub Actions runner has no default Git
identity, so the step failed with:

Git user.name is not set. Please configure git user.name globally,
locally or system wide.

Changes in build-reusable.yml

• Added Configure Git user step before Bump version (nebula final)
  that sets user.name and user.email to the standard
  github-actions[bot] identity
• Changed permissions.contents from read to write so the runner
  can push the version bump commit and tag back to the repo

[CLAassistant]

All committers have signed the CLA.

[leandrumartin]

Thank you for your contribution!