Revert "Drop conditionally applied cargo -Zon-broken-pipe=kill flags"

This reverts commit 5a7058c.

In [rust-lang#131059] we found out that `-Zon-broken-pipe=kill` is actually
**load-bearing** [1] for (at least) `rustc` and `rustdoc` to have the
kill-process-on-broken-pipe behavior, e.g. `rustc --print=sysroot |
false` will ICE and `rustdoc --print=sysroot | false` will panic on a
broken pipe.

[rust-lang#131059]: rust-lang#131059
[1]: rust-lang#131059 (comment)

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

@@ -1053,6 +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.
+    cargo.rustflag("-Zon-broken-pipe=kill");
+
     if builder.config.llvm_enzyme {
         cargo.rustflag("-l").rustflag("Enzyme-19");
     }

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

@@ -200,10 +200,6 @@ pub fn prepare_tool_cargo(
         cargo.arg("--features").arg(features.join(", "));
     }
 
-    // Warning: be very careful with RUSTFLAGS or command-line options, as conditionally applied
-    // RUSTFLAGS or cli flags can lead to hard-to-diagnose rebuilds due to flag differences, causing
-    // previous tool build artifacts to get invalidated.
-
     // Enable internal lints for clippy and rustdoc
     // NOTE: this doesn't enable lints for any other tools unless they explicitly add `#![warn(rustc::internal)]`
     // See https://github.com/rust-lang/rust/pull/80573#issuecomment-754010776
@@ -213,6 +209,13 @@ 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
+    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
 }