Avoid name colision when uploading artifact

We're seeing the following error when building multi-platform images
during upload artifact step:

"""
Failed to CreateArtifact: Received non-retryable error: Failed request: (409) Conflict: an artifact with this name already exists on the workflow run
"""
This PR fixes the issue by adding a built-in matrix enumeration to the
artifact name.

Context: actions/upload-artifact#480

Author: niconosenzo

.github/workflows/build.yaml

@@ -135,7 +135,7 @@ jobs:
       - name: Upload digest
         uses: actions/upload-artifact@v4
         with:
-          name: digests-${{ env.PLATFORM_PAIR }}
+          name: digests-${{ env.PLATFORM_PAIR }}-${{ strategy.job-index }}
           path: /tmp/digests/*
           if-no-files-found: error
           retention-days: 1