silence a warning about potential snprintf overrun

Some versions of gcc warn when building gen on 9.12 and 9.11 because an snprintf format can overrun the space available. The bug appears to have been fixed in 9.13 when the qname minimization work was merged, but it wasn't split out as a separate MR and backported. While looking at it I noticed that the size available for the directory name is too small anyway, so I increased it to PATH_MAX. Backporting this change will also silence the warning on 9.11.