Configure concurrency for e2e tests to fix flakiness (#530)

We experienced flakiness in the e2e tests after bumping the
revive-differential-tests submodule which included changes to the global
RPC concurrency limiter and batching layer. This PR attempts to fix the
flakiness by lowering the number of concurrent tasks spawned.

.github/workflows/differential-tests.yml

@@ -164,6 +164,9 @@ jobs:
           resolc-path: ./resolc-bin/resolc-x86_64-unknown-linux-musl
           solc-version: ${{ env.SOLC_VERSION }}
           expectations-file-path: ./revive/.github/assets/revive-dev-node-polkavm-resolc.json
+          # Using a higher concurrency, such as 100, currently overloads the dev node's RPC
+          # bridge, causing intermittent `eth_getTransactionReceipt` timeouts during code upload.
+          concurrency-number-of-concurrent-tasks: 50
 
   # Sentinel job that depends on the jobs we want to allow completing (instead of
   # being canceled once the other job chains complete). Thus, run-e2e-tests