Derive dir from filename if root is the same in winbuild

[radarhere]

Pillow/winbuild/build_prepare.py

Lines 134 to 135 in 4304414

	"filename": f"libjpeg-turbo-{V['JPEGTURBO']}.tar.gz",
	"dir": f"libjpeg-turbo-{V['JPEGTURBO']}",

It would seem like the "dir" could be derived from the "filename" most of the time, simply by removing the file extension.