Fix hang in --print=file-names in bootstrap

In an interactive context, the subprocess inherited a real tty stdin,
which lead it it waiting for something to happen, even though nothing
happened. By explicitly passing null as stdin we make sure an empty file
is passed, which achieves the desired behavior.

src/bootstrap/src/core/builder/cargo.rs

@@ -683,6 +683,7 @@ fn cargo(
                         .arg("--print=file-names")
                         .arg("--crate-type=proc-macro")
                         .arg("-")
+                        .stdin(std::process::Stdio::null())
                         .run_capture(self)
                         .stderr();
 

src/bootstrap/src/utils/exec.rs

@@ -120,6 +120,11 @@ pub fn current_dir<P: AsRef<Path>>(&mut self, dir: P) -> &mut Self {
         self
     }
 
+    pub fn stdin(&mut self, stdin: std::process::Stdio) -> &mut Self {
+        self.command.stdin(stdin);
+        self
+    }
+
     #[must_use]
     pub fn delay_failure(self) -> Self {
         Self { failure_behavior: BehaviorOnFailure::DelayFail, ..self }