Created by: await-ovo
Fixes #12177
When using npm 8, in some cases there are no postcss-flexbugs-fixes in the node_modules directory, instead they appear in node_modules/react-scripts/node_modules:
ls -l node_modules/react-scripts/node_modules | grep postcss
Therefore, we should use react-scripts' own postcss plugin dependencies, as well as postcss implementation.