Update Nix deps to incorporate latest Python packages

[rolfschr]

As described in the readme section "Keeping the Nix setup in sync", this PR updates the pypi index reference in order to accommodate for the recent updates of Python deps, mainly pygit2==1.5.0 which got released 2021-01-23.

Currently, the github workflow fails but this seems to be unrelated to Nix (alpine import error)

[InLaw]

e.g. black>19.3b0

• will break (currently) in nix (without patch) Black install via pip fails on embedded Python on Windows using latest pip version (20.3) psf/black#1847

but even with that version vulnerablecode will fail

• https://github.com/InLaw/vulnerablecode/actions/runs/632747085

error: --- Error --- nix
error: --- Error --- nix-daemon
builder for '/nix/store/hp49hf2d77xy30lg39klrxk5hslsjzvp-vulnerablecode-20.10-test.drv' failed with exit code 1; last 10 log lines:
      self.warn("could not create cache path {path}", path=path)
  
  /nix/store/c2yp70x0x41myhcqrzwfpc7x0q20c1rb-python3-3.8.5-env/lib/python3.8/site-packages/_pytest/cacheprovider.py:137
    /nix/store/c2yp70x0x41myhcqrzwfpc7x0q20c1rb-python3-3.8.5-env/lib/python3.8/site-packages/_pytest/cacheprovider.py:137: PytestCacheWarning: could not create cache path /nix/store/qlmcrzajxhskbcc3i2k81nyvxdbas18j-vulnerablecode-20.10/.pytest_cache/v/cache/lastfailed
      self.warn("could not create cache path {path}", path=path)

why do you cache in nix/store instead of e.g. /tmp?