Workaround for go-ethereum problem with gas estimation

When estimating gas price in `go-ethereum` ABI, `tx.gasprice` is not
passed to the function doing the estimation. This leads to incorrect gas
estimates when code execution path depends on `tx.gasprice`.

This was fixed today in
ethereum/go-ethereum#20189

We need to make a workaround in the contract code before this fix will
be released.

contracts/solidity/contracts/KeepRandomBeaconOperator.sol

@@ -471,7 +471,14 @@ contract KeepRandomBeaconOperator {
      * be returned to the DKG fee pool of the service contract which triggered the DKG.
      */
     function reimburseDkgSubmitter() internal {
-        uint256 gasPrice = tx.gasprice < priceFeedEstimate ? tx.gasprice : priceFeedEstimate;
+        uint256 gasPrice = priceFeedEstimate;
+        // We need to check if tx.gasprice is non-zero as a workaround to a bug
+        // in go-ethereum:
+        // https://github.com/ethereum/go-ethereum/pull/20189
+        if (tx.gasprice > 0 && tx.gasprice < priceFeedEstimate) {
+            gasPrice = tx.gasprice;
+        }
+
         uint256 reimbursementFee = dkgGasEstimate.mul(gasPrice);
         address payable magpie = stakingContract.magpieOf(msg.sender);