Handle build scripts better in -Zmacro-stats output.

Currently all build scripts are listed as `build_script_build` in the
stats header. This commit uses `CARGO_PKG_NAME` to improve that.

I tried it on Bevy, it works well, giving output like this on the build
script:
```
MACRO EXPANSION STATS: serde build script
```
and this on the crate itself:
```
MACRO EXPANSION STATS: serde
```

Author: nnethercote

compiler/rustc_interface/src/passes.rs

@@ -298,6 +298,16 @@ fn configure_and_expand(
 fn print_macro_stats(ecx: &ExtCtxt<'_>) {
     use std::fmt::Write;
 
+    let crate_name = ecx.ecfg.crate_name.as_str();
+    let crate_name = if crate_name == "build_script_build" {
+        // This is a build script. Get the package name from the environment.
+        let pkg_name =
+            std::env::var("CARGO_PKG_NAME").unwrap_or_else(|_| "<unknown crate>".to_string());
+        format!("{pkg_name} build script")
+    } else {
+        crate_name.to_string()
+    };
+
     // No instability because we immediately sort the produced vector.
     #[allow(rustc::potential_query_instability)]
     let mut macro_stats: Vec<_> = ecx
@@ -327,7 +337,7 @@ fn print_macro_stats(ecx: &ExtCtxt<'_>) {
     // non-interleaving, though.
     let mut s = String::new();
     _ = writeln!(s, "{prefix} {}", "=".repeat(banner_w));
-    _ = writeln!(s, "{prefix} MACRO EXPANSION STATS: {}", ecx.ecfg.crate_name);
+    _ = writeln!(s, "{prefix} MACRO EXPANSION STATS: {}", crate_name);
     _ = writeln!(
         s,
         "{prefix} {:<name_w$}{:>uses_w$}{:>lines_w$}{:>avg_lines_w$}{:>bytes_w$}{:>avg_bytes_w$}",