While yay seems to respect the $SRCDIR/$PKGDIR set in /etc/makepkg.conf, it can't find the built package when installing.
What version are you using? This was part of the template which you ignored :(
Sorry. Deleted too much of the template...
pacman -Qi yay
Name : yay
Version : 6.784-1
Probably also in some prior versions, I have this problem since a month or so at least.
You're running an old version. The pacman 5.1 update broke pkgdest support but it has since been fixed. You can either unset your pkgdest just for this one update or use makepkg directly.
Thank you, I reported this to Manjaro which still has the old one in the repos.