[20989] Set fallback branch for get_related_branch_from_repo correctly (backport #4816)

[mergify]

Description

Before this PR, when running the Ubuntu CI manually, if the input.fastdds-branch did not exist on ShapesDemo, the job fails, as the input.fastdds-branch is used as fallback branch for calling get_related_branch_from_repo for ShapesDemo.
In fact, it the PR is to an intermediate branch without a corresponding ShapesDemo branch, the job will also fail. The reason why it was not failing in normal PRs is that before the fallback check, the action checks for the existence on the base branch from the PR, and as ShapesDemo branching model is the same as Fast DDS' it was using the basing without having to check the fallback.

With this PR, the fallback branch is explicitly set on the reusable workflow so when running manual dispatching, the fallback branch is valid.

@Mergifyio backport 2.14.x 2.13.x 2.10.x 2.6.x

IMPORTANT: The fallback needs to be adjusted in the backports

Contributor Checklist

• Commit messages follow the project guidelines.

• N/A: The code follows the style guidelines of this project.

• 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: Any new/modified methods have been properly documented using Doxygen.

• N/A: Any new configuration API has an equivalent XML API (with the corresponding XSD extension)

• N/A: Changes are backport compatible: they do NOT break ABI nor change library core behavior.

• N/A: Changes are API compatible.

• N/A: New feature has been added to the versions.md file (if applicable).

• N/A: New feature has been documented/Current behavior is correctly described in the documentation.

• 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.

---

This is an automatic backport of pull request #4816 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e317234 has failed:

On branch mergify/bp/2.10.x/pr-4816
Your branch is up to date with 'origin/2.10.x'.

You are currently cherry-picking commit e3172347c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/reusable-ubuntu-ci.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[MiguelCompany]

LGTM with green Github Ubuntu CI (no-test)