You can put breakpoints in a task or a plugin, but not in your build scripts.
It takes some getting used to, but these days if I need to put a breakpoint in the build script, I take it as a “code smell” that my build is not declarative and I need to extract the imperative logic to a task or a plugin.
The easiest thing is to copy/paste the code in buildSrc and reference it from there. Then you can set your breakpoints, and attach your IDE on port 5005 after running Gradle like this:
gradlew -Porg.gradle.debug=true --no-daemon