opt-dist: dont overrwite config.toml when verifying

This is another step toward making opt-dist work in sandboxed environments

opt-dist verifies the final built rustc against a subset of rustc test
suite. However it overwrote the pre-existing `config.toml` [^1],
and that results in ./vendor/ directory removed [^2].

Instead of overwriting, this patch use `--set <config-value>` to
override paths to rustc / cargo / llvm-config.

[^1]: https://github.com/rust-lang/rust/blob/606afbb617a2949a4e35c4b0258ff94c980b9451/src/tools/opt-dist/src/tests.rs#L62-L77
[^2]: https://github.com/rust-lang/rust/blob/8679004993f08807289911d9f400f4ac4391d2bc/src/bootstrap/bootstrap.py#L1057

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

@@ -59,26 +59,17 @@ pub fn run_tests(env: &Environment) -> anyhow::Result<()> {
         .join(format!("llvm-config{}", executable_extension()));
     assert!(llvm_config.is_file());
 
-    let config_content = format!(
-        r#"profile = "user"
-change-id = 115898
+    let rustc = format!("build.rustc={}", rustc_path.to_string().replace('\\', "/"));
+    let cargo = format!("build.cargo={}", cargo_path.to_string().replace('\\', "/"));
+    let llvm_config =
+        format!("target.{host_triple}.llvm-config={}", llvm_config.to_string().replace('\\', "/"));
 
-[build]
-rustc = "{rustc}"
-cargo = "{cargo}"
-
-[target.{host_triple}]
-llvm-config = "{llvm_config}"
-"#,
-        rustc = rustc_path.to_string().replace('\\', "/"),
-        cargo = cargo_path.to_string().replace('\\', "/"),
-        llvm_config = llvm_config.to_string().replace('\\', "/")
-    );
-    log::info!("Using following `config.toml` for running tests:\n{config_content}");
+    log::info!("Set the following configurations for running tests:");
+    log::info!("\t{rustc}");
+    log::info!("\t{cargo}");
+    log::info!("\t{llvm_config}");
 
     // Simulate a stage 0 compiler with the extracted optimized dist artifacts.
-    std::fs::write("config.toml", config_content)?;
-
     let x_py = env.checkout_path().join("x.py");
     let mut args = vec![
         env.python_binary(),
@@ -97,6 +88,12 @@ llvm-config = "{llvm_config}"
         "tests/run-pass-valgrind",
         "tests/ui",
         "tests/crashes",
+        "--set",
+        &rustc,
+        "--set",
+        &cargo,
+        "--set",
+        &llvm_config,
     ];
     for test_path in env.skipped_tests() {
         args.extend(["--skip", test_path]);