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.`

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

@@ -1730,8 +1730,15 @@ 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 {
+                // If we're using `--stage 0`, we should provide the bootstrap cargo.
+                builder.initial_cargo.clone()
+            } else {
+                // 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 +2095,6 @@ 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 {