tweak the nix-shell for docs building

PR checklist:

[x] n/a Test coverage for the proposed changes
[x] n/a PR description contains example output from repl interaction or a snippet from unit test output
[x] n/a Documentation has been updated if new natives or FV properties have been added. To generate new documentation, issue cabal run tests. If they pass locally, docs are generated.
[x] n/a Any changes that could be relevant to users have been recorded in the changelog
[x] n/a In case of changes to the Pact trace output (pact -t), make sure pact-lsp is in sync.

Additionally, please justify why you should or should not do the following:

[x] n/a Confirm replay/back compat
[x] n/a Benchmark regressions
[x] n/a (For Kadena engineers) Run integration-tests against a Chainweb built with this version of Pact

kadena-io / pact