set on-broken-pipe in prepare_cargo_tool

Currently rustdoc breaks the build cache (due to having different rustflags) when building
rustdoc before building another tool (e.g., `x test miri && x test rustdoc && x test miri`).

This change fixes that by moving `on-broken-pipe` into `prepare_cargo_tool` so it is
set for all tools.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

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

@@ -211,6 +211,10 @@ pub fn prepare_tool_cargo(
     // See https://github.com/rust-lang/rust/issues/116538
     cargo.rustflag("-Zunstable-options");
 
+    // 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
 }
 
@@ -575,7 +579,9 @@ impl Step for Rustdoc {
             features.push("jemalloc".to_string());
         }
 
-        let mut cargo = prepare_tool_cargo(
+
+        // NOTE: Never modify the rustflags here, it breaks the build cache for other tools!
+        let cargo = prepare_tool_cargo(
             builder,
             build_compiler,
             Mode::ToolRustc,
@@ -586,11 +592,6 @@ impl Step for Rustdoc {
             features.as_slice(),
         );
 
-        // If the rustdoc 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");
-
         let _guard = builder.msg_tool(
             Kind::Build,
             Mode::ToolRustc,