Finn pip install overwrites existing finn-base install

[fpjentzsch]

Related Gitter discussion: https://gitter.im/xilinx-finn/community?at=60dad875effd491d8c8ec7bc

For some (?) users, the finn pip install in "finn_entrypoint.sh" (pip install --user -e $FINN_ROOT) re-installs finn-base from the official main branch even though a different finn-base installation (e.g. at a different commit hash or mounted from a local finn-base clone) already exists.

Finn-base is currently listed as dependency in "setup.cfg" to make the readthedocs build work: install_requires = finn-base @ git+https://github.com/Xilinx/finn-base#egg=finn-base

Curiously, this was never a problem for me; the finn-base requirement is satisfied by other versions without issue.