Avoid name colision when uploading artifact

[niconosenzo]

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 should be fixed by using the correct platform pair var names.

[graveland]

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

Was this due to retries? There should only be 2 jobs from the matrix, and they're differentiated by the platform name already? If it's due to retries, you might get > 2 digests to try to create a manifest with, which would also be a problem.

[niconosenzo]

Was this due to retries? There should only be 2 jobs from the matrix, and they're differentiated by the platform name already? If it's due to retries, you might get > 2 digests to try to create a manifest with, which would also be a problem.

😶‍🌫️ Just realized the problem is that I updated the var names and didn't really update them within the upload step:

          platform_slash_pair=linux/${{ matrix.platform }}
          echo "PLATFORM_DASH_PAIR=${platform_slash_pair//\//-}" >> $GITHUB_ENV