No, just tell the truth - you should never try to hide your true intentions from them. If you say tourism and then they ask you more questions, and they discover you are coming for a job interview instead, they can determine you are using deception to enter the UK (which can lead to a 10-year ban!).
There is nothing wrong with attending interviews in the UK as a visitor as long as you don't actually start working before you get a work visa. I have a friend who did this exact same thing (she attended 3 job interviews in a week and then flew back to the US) and she had no issues at all.
So, as long as you have proof of a return ticket, the job interview letter to show you've been invited, and evidence that you have a life to go back to in the US, there's no reason why you should have any issues.
Just to clarify though - it's immigration, not customs that you tell this to. The two are often confused, but customs has nothing to do with visas or immigration - customs is about import duties and tax, and so they only care about what items you are bringing into the country in your luggage. They need to make sure that you are not bringing in more than your allowed limits (i.e. of cash, alcohol, cigarettes, food etc.) and that you have declared anything you need to and have paid any relevant customs/import fees. If you aren't bringing anything you need to declare, you will likely just walk through the Green 'Nothing to Declare' line and probably won't even see a customs officer.