Pass bootstrap cargo when --stage 0 and COMPILETST_FORCE_STAGE0

And stop passing `BOOTSTRAP_CARGO` as an env var, instead the provided
cargo should go through `--cargo-path.`

Author: jieyouxu

src/bootstrap/src/core/build_steps/test.rs

@@ -1630,6 +1630,13 @@ struct Compiletest {
     compare_mode: Option<&'static str>,
 }
 
+/// `compiletest` under `--stage 0` causes `compiletest` to be used with the beta compiler which can
+/// cause many tests to fail. This env var is needed to indicate that the user understands the risk
+/// and actually intends to use `compiletest` with `--stage 0`.
+fn compiletest_force_stage0() -> bool {
+    env::var("COMPILETEST_FORCE_STAGE0").is_ok()
+}
+
 impl Step for Compiletest {
     type Output = ();
 
@@ -1647,7 +1654,7 @@ impl Step for Compiletest {
             return;
         }
 
-        if builder.top_stage == 0 && env::var("COMPILETEST_FORCE_STAGE0").is_err() {
+        if builder.top_stage == 0 && !compiletest_force_stage0() {
             eprintln!("\
 ERROR: `--stage 0` runs compiletest on the beta compiler, not your local changes, and will almost always cause tests to fail
 HELP: to test the compiler, use `--stage 1` instead
@@ -1730,8 +1737,16 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         let is_rustdoc = suite.ends_with("rustdoc-ui") || suite.ends_with("rustdoc-js");
 
         if mode == "run-make" {
-            let cargo = builder.ensure(tool::Cargo { compiler, target: compiler.host });
-            cmd.arg("--cargo-path").arg(cargo);
+            let cargo_path = if builder.top_stage == 0 && compiletest_force_stage0() {
+                // If we're forcing `--stage 0`, we should provide the bootstrap cargo.
+                builder.initial_cargo.clone()
+            } else {
+                // Unless we're forcing `--stage 0`, we need to properly build cargo using the
+                // suitable stage compiler.
+                builder.ensure(tool::Cargo { compiler, target: compiler.host })
+            };
+
+            cmd.arg("--cargo-path").arg(cargo_path);
         }
 
         // Avoid depending on rustdoc when we don't need it.
@@ -2088,8 +2103,6 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
             cmd.arg("--rustfix-coverage");
         }
 
-        cmd.env("BOOTSTRAP_CARGO", &builder.initial_cargo);
-
         cmd.arg("--channel").arg(&builder.config.channel);
 
         if !builder.config.omit_git_hash {