[Feature]: Update README install instructions with pipx

[iangetz]

Description

Related to: https://github.com/linode/linode-cli?tab=readme-ov-file#install

Current installation command:
pip3 install linode-cli

Recommended installation command:
pipx install linode-cli

Potentially also include upgrade command on README:
pipx upgrade linode-cli

Linux distributions such as Fedora and Debian now mark system Python environments as externally managed (PEP 668), which prevents or discourages installing packages globally with pip because doing so can overwrite OS-managed dependencies and potentially break system tools. pipx is designed specifically for installing Python-based CLI applications in isolated virtual environments, allowing users to install, upgrade, and remove tools like linode-cli without modifying the system Python environment. Recommending pipx aligns with current Python packaging best practices, avoids dependency conflicts, and ensures a safer and more predictable installation experience across modern Linux systems.

Thanks for your consideration! And a great CLI tool.

Potential Usage Example

No response

[CodeBySayak]

Hi! I’ve opened a PR to address this issue:
#877
Please let me know if any changes are needed. Thanks!