Use string, not repr, for log reporting PosixPath

Fix for #40002 (closed) introduced a small formatting issue in Chutney's log output, e.g.:

NOTE: creating PosixPath('/home/c/git/tor/../chutney/net/nodes.1596403717'), linking to PosixPath('/home/c/git/tor/../chutney/net/nodes')

Easy enough fix, I just need to make log output refer to the string expression of PosixPaths rather than the representation.