[windows] skip 'UNIX only'

# test that chezmoi does not panic if an external uses an absolute path
! exec chezmoi diff
stderr 'path is not relative'

-- home/user/.local/share/chezmoi/.chezmoiexternal.toml --
["/home/user/.dir"]
    type = "archive"
    url = "https://example.com/example.tar.gz"
