‘copy-recursively’ #:keep-mtime? #t broken for directories

> When passing #:keep-mtime? #t to ‘copy-recursively’, the mtime of

> regular files is preserved, but the mtime of directories (and symlinks)

> is not.

>

> This is because ‘set-file-time’ is called in the ‘down’ procedure of

> ‘file-system-fold’, before the directory is populated.

> ‘populate-store’ is affected by this bug, meaning that it creates stores

> where directories have “wrong” timestamps. In practice this is probably

> less likely to have undesirable side effects than regular files with

> wrong timestamps (think ‘make’, .go files, etc.)