Auto merge of #9405 - Hawk777:cargo-pkg-for-build-scripts, r=ehuss

Document that CARGO_PKG_ are availble to build.rs

Fixes GH-9403.

Author: bors

src/doc/src/reference/environment-variables.md

@@ -330,6 +330,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
                    changed by editing `.cargo/config.toml`; see the documentation
                    about [cargo configuration][cargo-config] for more
                    information.
+* `CARGO_PKG_<var>` - The package information variables, with the same names and values as are [provided during crate building][variables set for crates].
 
 [unix-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows
 [windows-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows
@@ -346,6 +347,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
 [jobserver]: https://www.gnu.org/software/make/manual/html_node/Job-Slots.html
 [cargo-config]: config.md
 [Target Triple]: ../appendix/glossary.md#target
+[variables set for crates]: #environment-variables-cargo-sets-for-crates
 
 ### Environment variables Cargo sets for 3rd party subcommands