Docs: Update installation instructions for PEP 668 (externally-managed-environment)

[rotirotirafa]

Hello LocalStack Team,

On modern Linux distributions (like Ubuntu 23.04+, Debian 12, etc.) that have implemented PEP 668, the current installation instruction pip install terraform-local will fail with an externally-managed-environment error.

This is because the OS now protects its system-level Python packages. (and that is great)

The recommended way to install Python-based command-line tools like terraform-local is by using pipx. This installs the tool in an isolated environment but makes the executable available globally, which i think is the perfect solution.

I suggest updating the README.md to recommend pipx as the primary installation method.