lfutil: provide a hint if the largefiles/lfs cache path cannot be determined A coworker hit this error using an LFS repo in a stripped down environment, and didn't know how to resolve it. The final conditional is a bit fast and loose, but there is currently no 'posix' test in hghave, and it doesn't seem like it's worth adding for this since I think Windows is the only non-POSIX platform we run tests on. Differential Revision: https://phab.mercurial-scm.org/D8145