core: copy CliqueConfig in DeveloperGenesisBlock (ethereum#23068)

Copy the CliqueConfig instead of reusing the pointer.
This makes DeveloperGenesisBlock thread safe and prevents it from
changing params.AllCliqueProtocolChanges.Clique.Epoch.

core/genesis.go

@@ -415,7 +415,10 @@ func DefaultCalaverasGenesisBlock() *Genesis {
 func DeveloperGenesisBlock(period uint64, faucet common.Address) *Genesis {
 	// Override the default period to the user requested one
 	config := *params.AllCliqueProtocolChanges
-	config.Clique.Period = period
+	config.Clique = &params.CliqueConfig{
+		Period: period,
+		Epoch:  config.Clique.Epoch,
+	}
 
 	// Assemble and return the genesis with the precompiles and faucet pre-funded
 	return &Genesis{