Yeah, I’m hitting this too with circleci/ruby:2.4.3-node-browsers and opened support ticket 31357 (no response yet).
/usr/local/bin/yarn is missing in the new image altogether, though yarn is under /opt/yarn-v1.5.1. Also /opt/yarn looks like was meant to be a symlink to /opt/yarn-v1.5.1 but is instead a dir containing a symlink.
Working image is circleci/ruby@sha256:970f014a06e9cf3bc1e3d5056ffde08da7467ac02e1ad86a47c32d78602ad6ed, and circleci/ruby@sha256:91348b8cd36263872b9c546ea568329bebcc1ba65fef06bee568fcb1aaba7322 is the new failing one (for 2.4.3-node-browsers).