Previously, `eval.full` organized the results for the supported systems
in a specific layout, i.e. with a folder with one subfolder per system.
Then, `eval.combine` relied on that.
When using `eval.singleSystem` and `eval.combine` directly, the caller
was responsible to recreate the same layout. This is annoying and
error-prone to do, when downloading artifacts from CI to recreate some
steps locally.
With this change, all the artifacts can be downloaded and extracted into
the same folder - because the result from `eval.singleSystem` already
contains the <system-name>/ subfolder.
eccd9564ab