ci/eval: improve api when calling in steps

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.

.github/workflows/eval.yml

@@ -95,6 +95,7 @@ jobs:
         with:
           pattern: intermediate-*
           path: intermediate
+          merge-multiple: true
 
       - name: Check out the PR at the test merge commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
@@ -158,6 +159,7 @@ jobs:
         with:
           name: result
           path: targetResult
+          merge-multiple: true
           github-token: ${{ github.token }}
           run-id: ${{ steps.targetRunId.outputs.targetRunId }}