Make rustc shim's verbose output include crate_name being compiled.

This change is mainly motivated by an issue with the environment
printing I added in PR 82403: multiple rustc invocations progress
in parallel, and the environment output, spanning multiple lines,
gets interleaved in ways make it difficult to extra the enviroment settings.

(This aforementioned difficulty is more of a hiccup than an outright
show-stopper, because the environment variables tend to be the same for all of
the rustc invocations, so it doesn't matter too much if one mixes up which lines
one is looking at. But still: Better to fix it.)

src/bootstrap/bin/rustc.rs

@@ -141,18 +141,23 @@ fn main() {
     if verbose > 1 {
         let rust_env_vars =
             env::vars().filter(|(k, _)| k.starts_with("RUST") || k.starts_with("CARGO"));
+        let prefix = match crate_name {
+            Some(crate_name) => format!("rustc {}", crate_name),
+            None => "rustc".to_string(),
+        };
         for (i, (k, v)) in rust_env_vars.enumerate() {
-            eprintln!("rustc env[{}]: {:?}={:?}", i, k, v);
+            eprintln!("{} env[{}]: {:?}={:?}", prefix, i, k, v);
         }
-        eprintln!("rustc working directory: {}", env::current_dir().unwrap().display());
+        eprintln!("{} working directory: {}", prefix, env::current_dir().unwrap().display());
         eprintln!(
-            "rustc command: {:?}={:?} {:?}",
+            "{} command: {:?}={:?} {:?}",
+            prefix,
             bootstrap::util::dylib_path_var(),
             env::join_paths(&dylib_path).unwrap(),
             cmd,
         );
-        eprintln!("sysroot: {:?}", sysroot);
-        eprintln!("libdir: {:?}", libdir);
+        eprintln!("{} sysroot: {:?}", prefix, sysroot);
+        eprintln!("{} libdir: {:?}", prefix, libdir);
     }
 
     let start = Instant::now();