[python-package] ensure 'build-python.sh' always reinstalls (fixes #5906)

[jameslamb]

Fixes #5906.

On master, build-python.sh install will not update the installed version of lightgbm if that version has already been installed. For example:

sh build-python.sh install
sh build-python.sh install
# Requirement already satisfied: lightgbm in  ../mambaforge/lib/python3.9/site-packages (3.3.5.99)

This PR proposes modifying build-python.sh such that it'll always install the version it's just built, regardless of what's already installed. I think that is closer to what is expected when building from sources cloned from GitHub, and it also makes local development a bit more reliable and convenient (no need to pip uninstall each time you rebuild).

Notes for Reviewers

The relevant code for installing with --precompile doesn't need to change, as python setup.py install will always reinstall over an existing version.

LightGBM/build-python.sh

Line 307 in ad487fe

python setup.py install ${PIP_INSTALL_ARGS} --precompile

[github-actions]

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