Work-around lack of artifact dependencies when using cargo run -p bootstrap

While working on rust-lang#94806, I was very confused
because the `trim()` fix didn't actually do anything and still errored out.
It turns out the issue was that bootstrap had been rebuilt, but not fake rustc.
Rebuild all the fake binaries automatically from within bootstrap to avoid issues like this in the future.

Author: jyn514

src/bootstrap/lib.rs

@@ -442,12 +442,7 @@ impl Build {
             let workspace_target_dir = std::env::var("CARGO_TARGET_DIR")
                 .map(PathBuf::from)
                 .unwrap_or_else(|_| src.join("target"));
-            let bootstrap_out = workspace_target_dir.join("debug");
-            if !bootstrap_out.join("rustc").exists() {
-                // this restriction can be lifted whenever https://github.com/rust-lang/rfcs/pull/3028 is implemented
-                panic!("run `cargo build --bins` before `cargo run`")
-            }
-            bootstrap_out
+            workspace_target_dir.join("debug")
         };
 
         let mut build = Build {
@@ -490,6 +485,11 @@ impl Build {
             tool_artifacts: Default::default(),
         };
 
+        if env::var("BOOTSTRAP_PYTHON").is_err() {
+            // This can be removed whenever https://github.com/rust-lang/rfcs/pull/3028 is implemented.
+            build.run(Command::new(env!("CARGO")).args(&["build", "-p", "bootstrap", "--bins"]));
+        }
+
         build.verbose("finding compilers");
         cc_detect::find(&mut build);
         // When running `setup`, the profile is about to change, so any requirements we have now may