bootstrap: Move RUSTC_BREAK_ON_ICE out of shim

This is always set, so let's just always set it elsewhere to reduce the
need for our `rustc.rs` shim.

Author: alexcrichton

src/bootstrap/bin/rustc.rs

@@ -97,8 +97,6 @@ fn main() {
         cmd.env("RUST_BACKTRACE", "1");
     }
 
-    cmd.env("RUSTC_BREAK_ON_ICE", "1");
-
     if let Ok(debuginfo_level) = env::var("RUSTC_DEBUGINFO_LEVEL") {
         cmd.arg(format!("-Cdebuginfo={}", debuginfo_level));
     }

src/bootstrap/builder.rs

@@ -917,7 +917,8 @@ impl<'a> Builder<'a> {
                     PathBuf::from("/path/to/nowhere/rustdoc/not/required")
                 },
             )
-            .env("RUSTC_ERROR_METADATA_DST", self.extended_error_dir());
+            .env("RUSTC_ERROR_METADATA_DST", self.extended_error_dir())
+            .env("RUSTC_BREAK_ON_ICE", "1");
 
         if let Some(host_linker) = self.linker(compiler.host) {
             cargo.env("RUSTC_HOST_LINKER", host_linker);