(maint) Update PMT to work around cross-platform issues in Dir.[]

The (unspecified) default behavior of the Dir.[] method, which will implicitly coerce non-String arguments into Strings. The win32-dir gem
overrides this method and calls String specific methods on the arguments
before any conversions are done. This change makes our use of the
method in question more explicit, passing Strings instead of Pathname
objects to avoid the interpreter's ire.