Atomicfu: Fix test environment setup

[Soarex16]

Currently, tests for the atomicfu plugin give the following error when run:

FAILURE: Build failed with an exception.
* What went wrong:
Could not determine the dependencies of task ':kotlinx-atomicfu-compiler-plugin:test'.
> Task with path ':js:js.tests:unzipV8' not found in project ':kotlinx-atomicfu-compiler-plugin'.

As you can see, this problem occurred because the build script refers to a non-existent task unzipV8 in the :js:js.tests module. Judging by the commit history, the cause of the problem is a change in the way to download V8 made by dab1ec7affd261a42165fded8a29b52ca620a6da.

This fix simply adapts the build script to the changes in the V8 setup.

[Soarex16]

@mvicsokolova, shouldn't it be merged? 🤔