txnbuild: multiple calls to Transaction.build() increments operations list

[poliha]

Lets say you have the following transaction

	tx := Transaction{
		SourceAccount: &sourceAccount,
		Operations:    []Operation{&createAccount},
		Timebounds:    NewInfiniteTimeout(),
		Network:       network.TestNetworkPassphrase,
	}

	err := tx.Build()
	err = tx.Build()

After calling tx.Build() twice, the transaction will have 2 operations instead of one. The intended behaviour is for the transaction to have just one operation.
Also, multiple calls to build will increment the sequence number multiple times, which is not what is probably desired.