fix: Avoid duplicated "Change Log" file header on Windows due to CRLF (closes #710)

[Gustl22]

Closes #710

Type of Change

• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• ❌ ! -- Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 refactor -- Code refactor
• ✅ ci -- Build configuration change
• 📝 docs -- Documentation
• 🗑️ chore -- Chore

[docs-page]

To view this pull requests documentation preview, visit the following URL:

docs.page/invertase/melos~915

Documentation is deployed and generated using docs.page.

[spydon]

Super, thanks!

[Gustl22]

I didn't write a test. I suppose it would be something like

    test('AggregateChangelog', () async {
      final changelog = AggregateChangelog(
        testWorkspace,
        config.description,
        dateSlug,
        pendingPackageUpdates,
        logger,
        config.path,
      );

      final read = await changelog.read();
      expect(read, '''
# Change Log
...etc.
''');
    });

Not quite sure how to mock these things and where to put this.