chore(ci): Disable nightly CI runs

[LucioFranco]

Starting to see nightly fail with:

error[E0277]: `impl Send` is not a future
  --> examples/examples/tower-balance.rs:109:19
   |
109 |   fn gen_disco() -> impl Discover<
   |  ___________________^
110 | |     Key = Key,
111 | |     Error = Error,
112 | |     Service = ConcurrencyLimit<
113 | |         impl Service<Req, Response = Rsp, Error = Error, Future = impl Send> + Send,
114 | |     >,
115 | | > + Send {
   | |________^ `impl Send` is not a future
   |
   = help: the trait `std::future::Future` is not implemented for `impl Send`
   = note: impl Send must be a future or must implement `IntoFuture` to be awaited
   = note: required because of the requirements on the impl of `Service<Req>` for `ServiceFn<[closure@examples/examples/tower-balance.rs:121:45: 121:53]>`
   = note: required because of the requirements on the impl of `futures_core::stream::TryStream` for `Disco<ConcurrencyLimit<ServiceFn<[closure@examples/examples/tower-balance.rs:121:45: 121:53]>>>`
   = note: required because of the requirements on the impl of `tower::discover::Discover` for `Disco<ConcurrencyLimit<ServiceFn<[closure@examples/examples/tower-balance.rs:121:45: 121:53]>>>`

Which is a regression probably related to rust-lang/rust#99536, I think for now its fine to disable nightly while keeping beta and stable for version checks.