Force use of Yarn Classic to build Anki

[dae]

I had quite a trouble trying to build Anki, and the cause was because Yarn Modern (Berry) was being used instead of Yarn Classic (1.x).
At first, I didn’t realize that wrong Yarn version was the cause. The build fails while building canvas@npm:1.0.0 package, with the message that 'cairo/cairo.h' file not found.

Originally reported on https://forums.ankiweb.net/t/use-yarn-classic-to-build-anki/52919/4

Possibly solution: https://forums.ankiweb.net/t/use-yarn-classic-to-build-anki/52919/5?u=dae