Simulator builds fail due to use of stable release channel

[garethknowles]

I'm getting failing simulator builds of the crankstart examples.
The device builds (eg --device) are fine.

I'm new to both playdate & rust so bare with me. I've followed the full setup in the README including the nightly toolchain and thumbv7em-none-eabihf target. I'm on macOS 13.6.6.

Here's the full error:

crank build --release --example hello_world
   Compiling num-traits v0.2.18
   Compiling cstr_core v0.1.2
   Compiling rand v0.8.5
   Compiling rand_pcg v0.3.1
   Compiling randomize v3.0.1
   Compiling syn v1.0.109
   Compiling hashbrown v0.14.3
error[E0554]: `#![feature]` may not be used on the stable release channel
  --> /Users/garethknowles/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cstr_core-0.1.2/src/lib.rs:12:40
   |
12 | #![cfg_attr(feature = "alloc", feature(alloc))]
   |                                        ^^^^^

For more information about this error, try `rustc --explain E0554`.
error: could not compile `cstr_core` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
Error: cargo failed with error ExitStatus(unix_wait_status(25856))

[garethknowles]

Adding +nightly flag to the simulator builds - matching what seems to be done for device builds looks to fix the issue for me locally. I don't really know what I'm doing here but I've go the examples running now 👍

pd-rs/crank/pull/58