rz_core_task_join: Return true on join

[kazarmy]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

This pr makes rz_core_task_join() return true if the join happened (completely). This is part of efforts to figure out why the woodpecker build is failing.

Test plan

All builds except the woodpecker build are green.

Closing issues

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