Rollup merge of #142091 - thaliaarchi:aix-getenv, r=workingjubilee

Fix AIX build

Fix rust-lang/rust#141543.

`getenv` was moved out of this file to `sys::env::getenv` in rust-lang/rust#140143. Replace its usage with `std::env::var_os`, the publicly exposed version. This matches the other usages of the same function in this file.

Author: matthiaskrgr