fix: reuse single Manifest instance for releases and pull requests

[omerarslan0]

The main() function instantiated two separate Manifest objects: one for creating releases and another for creating pull requests. Each call to loadOrBuildManifest() triggers GitHub API requests to fetch repository configuration and release history.

When draft: true is enabled this redundancy becomes especially costly because draft release resolution requires iterating through more GitHub API pages to find tagged releases, effectively doubling the API overhead.

This PR hoists the loadOrBuildManifest() call above both conditional blocks so a single, shared instance is used for both operations.

Originally opened against the upstream googleapis/release-please-action in #1190; rebased here per maintainer request.

Test plan

• npm test passes
• npm run build regenerates dist/index.js
• npm run lint and npm run fmt:check pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[yafanasiev]

@omerarslan0 thank you!