lookup: add pnpm

[GeoffreyBooth]

Split off from #1020.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (46d35ae) 96.32% compared to head (c2e4677) 96.32%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1027   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files          28       28           
  Lines        2177     2177           
=======================================
  Hits         2097     2097           
  Misses         80       80           

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[GeoffreyBooth]

The workflow that ran for this PR passed with a retry for v18 Windows, but didn’t run for 21 (?). When I run it manually I get failures: https://github.com/nodejs/citgm/actions/runs/6843202973/job/18605533429. Are the differing results due to flakiness or something about how the workflows are run?