feat(opt-dist): respect existing .cargo/config.toml

Respect `.cargo/config.toml` in the rustc source.
This is useful when the source is from a tarball
and contains vendored source settings.

src/tools/opt-dist/src/training.rs

@@ -36,7 +36,7 @@ fn init_compiler_benchmarks(
     // Run rustc-perf benchmarks
     // Benchmark using profile_local with eprintln, which essentially just means
     // don't actually benchmark -- just make sure we run rustc a bunch of times.
-    cmd(&[
+    let mut cmd = cmd(&[
         env.cargo_stage_0().as_str(),
         "run",
         "-p",
@@ -61,7 +61,16 @@ fn init_compiler_benchmarks(
     .env("RUST_LOG", "collector=debug")
     .env("RUSTC", env.rustc_stage_0().as_str())
     .env("RUSTC_BOOTSTRAP", "1")
-    .workdir(&env.rustc_perf_dir())
+    .workdir(&env.rustc_perf_dir());
+
+    // Respect `.cargo/config.toml` in the rustc source. This is useful when the
+    // source is from a tarball and contains vendored source settings.
+    let dot_cargo_config_toml = env.checkout_path().join(".cargo").join("config.toml");
+    if dot_cargo_config_toml.is_file() {
+        cmd.arg("--cargo-config").arg(dot_cargo_config_toml);
+    }
+
+    cmd
 }
 
 /// Describes which `llvm-profdata` binary should be used for merging PGO profiles.