Scan file descriptors rather than files per go-yara docs

[egibs]

This is a quick PR to swap over from scanning files to scanning file descriptors.

go-yara's documentation mentions that this is the better route in a comment above ScanFile:

// ScanFile scans a file using the ruleset. For every
// event emitted by libyara, the corresponding method on the
// ScanCallback object is called.
//
// Note that the filename is passed as-is to the YARA library and may
// not be processed in a sensible way. It is recommended to avoid this
// function and to obtain an os.File handle f using os.Open() and use
// ScanFileDescriptor(f.Fd(), …) instead.

In my testing, this shaved off several seconds of overhead.