Determine rust channel by parsing rustc output

The RUSTUP_TOOLCHAIN environment variable might not always be present.
This is the case for e.g. NixOS where rust is routinely not installed via
rustup, thus not setting this env var, causing build failures.
Instead, build.rs will now run `rustc -V` and check if the output contains the
word "nightly".

azalea/build.rs

@@ -1,8 +1,12 @@
-use std::env;
+use std::process::Command;
 
 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.");
+    let rustc_version_output = Command::new("rustc").arg("-V").output().unwrap();
+    if !rustc_version_output.status.success()
+        || !String::from_utf8(rustc_version_output.stdout)
+            .unwrap()
+            .contains("nightly")
+    {
+        panic!("Azalea currently requires nightly Rust. If you have installed Rust with rustup you can use `rustup override set nightly` to set the toolchain for this directory.");
     }
 }