rustFlake: doesn't fail on reported cargo clippy AND cargo build warnings

[bladyjoker]

Original: mlabs-haskell/lambda-buffers#165

warning: very complex type used. Consider factoring parts into `type` definitions
  --> src/indexer/callback.rs:22:5
   |
22 |     Arc<dyn Fn(Event) -> Pin<Box<dyn Future<Output = Result<(), E>> + Send + Sync>> + Send + Sync>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_complexity
   = note: `#[warn(clippy::type_complexity)]` on by default
warning: using `Result.or_else(|x| Err(y))`, which is more succinctly expressed as `map_err(|x| y)`
  --> src/indexer/callback.rs:43:9
   |
43 | /         rt.block_on(handle_event(input, |ev: Event| f(ev), &retry_policy, utils))
44 | |           .or_else(|err| {
45 | |             event!(Level::ERROR, label=%Events::EventHandlerFailure, ?err);
46 | |             Err(err)
47 | |           })
   | |____________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#bind_instead_of_map
   = note: `#[warn(clippy::bind_instead_of_map)]` on by default
help: try
   |
44 ~           .map_err(|err| {
45 |             event!(Level::ERROR, label=%Events::EventHandlerFailure, ?err);
46 ~             err
   |

[bladyjoker]

@szg251 is there an easy way to fail builds when any warning is issued? That's what we do in other places.