also set `$CARGO_HTTP_CAINFO` on RHEL systems

[bedroge]

While debugging a Rust problem, I just ran into this error on a Rocky 8 node:

$ cargo add bitvec
    Updating crates.io index
error: download of config.json failed

Caused by:
  failed to download from `https://index.crates.io/config.json`

Caused by:
  [77] Problem with the SSL CA cert (path? access rights?) (error setting certificate file: /etc/ssl/certs/ca-certificates.crt)

We already set a bunch of variables on RHEL-based systems, see:
https://github.com/EESSI/software-layer-scripts/blob/main/init/modules/EESSI/2023.06.lua#L286

But for Cargo we also need to set $CARGO_HTTP_CAINFO to the same path; that solved the issue for me:

$ export CARGO_HTTP_CAINFO=/etc/pki/tls/certs/ca-bundle.crt
$ cargo add bitvec
    Updating crates.io index
      Adding bitvec v1.1.1 to dependencies
             Features:
             + alloc
             + atomic
             + std
             - serde
             - testing
    Updating crates.io index
     Locking 5 packages to latest Rust 1.91.1 compatible versions