libressl: fixing nc for version>=2.9

This addresses https://github.com/NixOS/nixpkgs/issues/68286

When `-R` (CA file location) is not specified, nc tries to fall back to
a default location. In 2.8 this was still configurable at compile time,
but was changed somewhere after. This replaces `/etc/ssl/cert.pem`
with `${cacert}/etc/ssl/cert.pem` in the code directly.

For a discussion of this, see https://github.com/NixOS/nixpkgs/pull/68456