pip install --no-build-isolation
https://pip.pypa.io/en/stable/cli/pip_install/#cmdoption-no-build-isolation
#pip_install
Disable isolation when building a modern source distribution. Build dependencies specified by PEP 518 must be already installed if this option is used.
（積ん読）PEP 518 – Specifying Minimum Build System Requirements for Python Projects
flash-attn
で見た