bootstrap will detect the host triple as `HOST`, which is set by the `cargo`
that's invoking `bootstrap`. `cargo` autodetects `HOST` from the `rustc` it has
available (e.g., rust-bootstrap's). The `rustc` from `rust-bootstrap` has
`x86_64-unknown-linux-gnu` hardcoded as its host.

To break this cycle, hardcode x86_64-pc-linux-gnu here.

diff --git a/src/bootstrap/build.rs b/src/bootstrap/build.rs
index cd1f41802..33cedd624 100644
--- a/src/bootstrap/build.rs
+++ b/src/bootstrap/build.rs
@@ -1,7 +1,4 @@
-use std::env;
-
 fn main() {
-    let host = env::var("HOST").unwrap();
     println!("cargo:rerun-if-changed=build.rs");
-    println!("cargo:rustc-env=BUILD_TRIPLE={}", host);
+    println!("cargo:rustc-env=BUILD_TRIPLE=x86_64-pc-linux-gnu");
 }