`buck audit cell` returns nonexistent paths for external cells

[lf-]

Repro on buck2 2026-07-14-1560aca2002865cd73d7cafb22c705cfb640b2bc: dump this in .buckconfig.local in the buck2 oss repo:

[cells]
buck2-haskell = buck2-haskell

[external_cells]
buck2-haskell = git

[external_cell_buck2-haskell]
git_origin = https://github.com/MercuryTechnologies/buck2-haskell.git
commit_hash = ead8e268c37dec76c273d298e1e8c2cb1520cffa

$ ./bootstrap/buck2 audit cell
File changed: gh_facebook_buck2//.buckconfig.local
Build ID: ebec6bac-75a7-4a0a-83bd-afbc8c8b3e95
Command: audit.                                                                                                                                  
Time elapsed: 0.1s
prelude: /Users/jade/co/buck2/prelude
buck2-haskell: /Users/jade/co/buck2/buck2-haskell
                         ^^^^^^^^
gh_facebook_buck2_shims_meta: /Users/jade/co/buck2/shim
gh_facebook_buck2: /Users/jade/co/buck2
none: /Users/jade/co/buck2/none

This is part of a broader problem, which is that there's not an obvious command to get the path of an external cell, and there probably needs to be. My agent thought that buck root prelude did it (nope), and that buck audit cell did it (nope, but with falsehoods).