tweak the nix-shell for docs building

[imalsogreg]

PR checklist:

• n/a Test coverage for the proposed changes
• n/a PR description contains example output from repl interaction or a snippet from unit test output
• 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.
• n/a Any changes that could be relevant to users have been recorded in the changelog
• 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:

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

[sirlensalot]

Works in my env. @jmcardon or @emilypi wanna confirm/approve?

[emilypi]

Confirmed for working on my machine