Change default python branch from 'main' to 'master'

[MiguelCompany]

Description

Due to the change of the default branch in Fast DDS Python repo, we need to change the fallback branch for that repo in the CI of this one.

Contributor Checklist

• NO: Commit messages follow the project guidelines.
• N/A: Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A: New feature has been documented/Current behavior is correctly described in the documentation.
• N/A: Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.