Rollup merge of #143228 - nnethercote:macro-stats-build-scripts, r=Kobzol

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
```

r? `@Kobzol`

Author: matthiaskrgr