The only way I found to answer your question was to reduce the input to provide to the copy task to a single file ditching in the process all references to a tree structure.
plugins {
id("base") // to get access to the clean task
}
task unzip(type: Copy) {
// since your zip is in the buildDir, I suppose you'll need deferred evaluation, hence the from(Closure) variant
from {
zipTree("$buildDir/lib/test.zip").matching { include '**/result1.jar' }.singleFile
}
into buildDir
}