Set BUILD_TRIPLE via build script

This moves build triple discovery for rustbuild from bootstrap.py into a build
script, meaning it will "just work" if building rustbuild via Cargo rather than
Python.

src/bootstrap/Cargo.toml

@@ -3,6 +3,7 @@ authors = ["The Rust Project Developers"]
 name = "bootstrap"
 version = "0.0.0"
 edition = "2018"
+build = "build.rs"
 
 [lib]
 path = "lib.rs"

src/bootstrap/bootstrap.py

@@ -1032,7 +1032,6 @@ def bootstrap(help_triggered):
     args = [build.bootstrap_binary()]
     args.extend(sys.argv[1:])
     env = os.environ.copy()
-    env["BUILD"] = build.build
     env["SRC"] = build.rust_root
     env["BOOTSTRAP_PARENT_ID"] = str(os.getpid())
     env["BOOTSTRAP_PYTHON"] = sys.executable

src/bootstrap/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("cargo:rustc-env=BUILD_TRIPLE={}", std::env::var("HOST").unwrap());
+}

src/bootstrap/config.rs

@@ -487,7 +487,7 @@ impl Config {
         config.missing_tools = false;
 
         // set by bootstrap.py
-        config.build = TargetSelection::from_user(&env::var("BUILD").expect("'BUILD' to be set"));
+        config.build = TargetSelection::from_user(&env!("BUILD_TRIPLE"));
         config.src = Config::path_from_python("SRC");
         config.out = Config::path_from_python("BUILD_DIR");