perf test: fix snap artifact names

By mistake, we're using the same name for both snap artifacts,
which is why the snap upload fails.

This commit will update the artifact names to match the snap file
names.

.github/workflows/performance.yaml

@@ -146,11 +146,11 @@ jobs:
         if: failure()
         uses: actions/upload-artifact@v4
         with:
-          name: base
+          name: base-code.snap
           path: ${{ github.workspace }}/base-code.snap
       - name: Upload head.snap
         if: failure()
         uses: actions/upload-artifact@v4
         with:
-          name: base
+          name: head.snap
           path: ${{ github.workspace }}/head.snap