propagate strip_top_level_dir option to slapos.recipe.build:downloadunpacked

Don't force strip_top_level_dir option to be false when calling for slapos.recipe.build:downloadunpacked - let the user decide.

Effectively adds strip_top_level_dir option to the script which allows for shorter and more consistent paths. Also it can help mitigating the variability of path length between archives downloaded for branch, commit hash and release.