Use untracked env var to pass -Zon-broken-pipe=kill for tools

- Don't touch rustc's `-Zon-broken-pipe=kill` env var in `compile.rs`.
- Use an untracked env var to pass `-Zon-broken-pipe=kill` for tools but
  skip cargo still, because cargo wants `-Zon-broken-pipe=kill` unset.

Author: jieyouxu

src/bootstrap/src/bin/rustc.rs

@@ -137,6 +137,10 @@ fn main() {
         cmd.args(lint_flags.split_whitespace());
     }
 
+    if let Ok(flags) = env::var("UNTRACKED_BROKEN_PIPE_FLAG") {
+        cmd.args(flags.split_whitespace());
+    }
+
     if target.is_some() {
         // The stage0 compiler has a special sysroot distinct from what we
         // actually downloaded, so we just always pass the `--sysroot` option,

src/bootstrap/src/core/build_steps/compile.rs

@@ -1053,8 +1053,10 @@ pub fn rustc_cargo(
 
     cargo.rustdocflag("-Zcrate-attr=warn(rust_2018_idioms)");
 
-    // If the rustc output is piped to e.g. `head -n1` we want the process to be
-    // killed, rather than having an error bubble up and cause a panic.
+    // If the rustc output is piped to e.g. `head -n1` we want the process to be killed, rather than
+    // having an error bubble up and cause a panic.
+    // NOTE(jieyouxu): this flag is load-bearing for rustc to not ICE on broken pipes.
+    // See <https://github.com/rust-lang/rust/issues/131059> for details.
     cargo.rustflag("-Zon-broken-pipe=kill");
 
     if builder.config.llvm_enzyme {

src/bootstrap/src/core/build_steps/tool.rs

@@ -209,11 +209,12 @@ pub fn prepare_tool_cargo(
     // See https://github.com/rust-lang/rust/issues/116538
     cargo.rustflag("-Zunstable-options");
 
-    // `-Zon-broken-pipe=kill` breaks cargo tests
+    // cargo explicitly does not want `-Zon-broken-pipe=kill` semantics, and thankfully cargo tests
+    // will break if we do set the flag. We use a special env var here that is not part of
+    // `RUSTFLAGS` tracked by cargo as that can cause unnecessary tool rebuilds due to tool build
+    // cache invalidation.
     if !path.ends_with("cargo") {
-        // If the output is piped to e.g. `head -n1` we want the process to be killed,
-        // rather than having an error bubble up and cause a panic.
-        cargo.rustflag("-Zon-broken-pipe=kill");
+        cargo.env("UNTRACKED_BROKEN_PIPE_FLAG", "-Zon-broken-pipe=kill");
     }
 
     cargo