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`
f2231d5324