fix: support relative downloadsPath directory for downloads

[mxschmitt]

Before this change there were two issues:

• On Windows it was not working at all when a relative download path was passed
• Artifact.path() -> Download.path() returned the relative path + uuid, it returns now it's absolute variant to be consistent with the behavior when an absolute path was given

Fixes #6401

[dgozman]

I'd rather resolve the path in validateLaunchOptions and assume absolute path throughout the rest of the code. This way we won't need any changes in Artifact or launch.