chore(ci): Reuse built extension on tests

[JoseLion]

As discussed in #1419:

A better way to run CI is to build the extension and test it on different Java versions. We can leverage the artifacts already built on the build-and-analyse job to do that. We upload the required artifacts of the built extension and then download them to be reused on the test-extension job.

[jdneo]

Looks like the errors are different on different platform. Some path related issues.

On windows, it's ENOENT: no such file or directory, mkdir 'D:\a\vscode-gradle\vscode-gradle\extension\lib

On Mac/Linux, it's chmod: cannot access 'extension/lib/gradle-server': No such file or directory.

[JoseLion]

Yes, I misunderstood the documentation of the download-artifact action, so it was downloading in the incorrect path. I pushed a commit with a fix 🙂

[JoseLion]

One more thing's missing 😅 even though we don't need to run ./gradlew extension:build anymore, we still need to install NPM dependencies in the test-extension job. I'll push a fix for that too 🙂

[jdneo]

Fantastic! It's not an easy task to update such a complex build description file.

Thank you for the contribution!