linkchecker: fix bad interaction with python3
linkchecker: fix bad interaction with python3

Previously, `nix-shell -p linkchecker python3` would lead to a broken
`python3` command. That's because Python 2 packages would pollute the
environment and confuse the interpreter. `linkchecker` has propagated
build inputs. So the solution is to use `pythonPath` instead, to avoid
the pollution.

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