Replace now non-existent humble branch in flatland repo

[glpuga]

Proposed changes

The ros2-humble branch we used to pull from the flatland repo no longer exists, and CI fails. Fix by replacing with hash.

Type of change

• 🐛 Bugfix (change which fixes an issue)
• 🚀 Feature (change which adds functionality)
• 📚 Documentation (change which fixes or extends documentation)

💥 Breaking change! Explain why a non-backwards compatible change is necessary or remove this line entirely if not applicable.

Checklist

Put an x in the boxes that apply. This is simply a reminder of what we will require before merging your code.

• Lint and unit tests (if any) pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• All commits have been signed for DCO

Additional comments

Anything worth mentioning to the reviewers.