Fix Yarn diagnose report support by not assuming `node_modules` exists

[unflxw]

The diagnose report, when executed via npx @appsignal/cli@latest diagnose (which is the currently recommended way to execute it) checks for the presence of a node_modules in the current folder, or any folders above it, and looks for @appsignal/nodejs within it, in order to require the actual implementation of the diagnose report from it.

This is incompatible with package managers such as Yarn, which do not store dependencies in a node_modules folder at the root of the project.