Fix windows build

[JeremieSpiesser]

Hello :wave:, I had two errors when building sanzu for windows.

1. 404 error when running pacman -Syu
2. Error because the tarball of ffmpeg was removed from the repo (due to the xz drama)

This PR makes it possible to build again for windows by

1. Changing the archlinux mirror used
2. Updating the ffmpeg tarball

On the "latest" channel, there is no n4.x version anymore for ffmpeg so I used version n6.1. It seems to work fine from my limited testing (machine with an intel gpu using h264_qsv)

[serpilliere]

Hi @JeremieSpiesser !

Thank you for the fix. We were aware of the ffmpeg version, but made some "time delay" for some old OS (think Centos7). But it may be time to update it now, as we are reaching the EOL.

Can you signoff the commit to pass the DCO?

[serpilliere]

Thank you for the fix @JeremieSpiesser !