fix: install pipx using brew and apt to unblock use with python 3.14

[ocofaigh]

Description

In python 3.14 they don't allow you to use pip globally and the install of pipx was failing when using python3.14 pip.
To fix this, I have updated to remove all dependencies on pip, and used brew (for mac) and apt (for debian / ubuntu) to install pipx instead

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