show error if user is on stable rust

Author: mat-1

azalea/build.rs

@@ -0,0 +1,8 @@
+use std::env;
+
+fn main() {
+    let rust_toolchain = env::var("RUSTUP_TOOLCHAIN").unwrap();
+    if rust_toolchain.starts_with("stable") {
+        panic!("Azalea currently requires nightly Rust. You can use `rustup override set nightly` to set the toolchain for this directory.");
+    }
+}