Problem:
stdpath() may return a DOS 8.3 "shortened" filename, because Windows
truncates long usernames into `6ch~N` names in `TEMP/TMP` env vars.
We don't want to "leak" them into Nvim.
Solution:
For "run", pass `true` to `vim_FullName` to expand 8.3 filenames.
For "cache", call `os_realpath` to expand 8.3 filenames.
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>