dist.py: Make work with new win sysroot layout
dist.py: Make work with new win sysroot layout

Adopts the script to work after the changes in:
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2372727
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2655836

After those, the paths in the json file are relative to the root,
and the windows sdk is in "Windows Kits/10" instead of in "win_sdk".

nico/demumble,nico/demumble