racket{,-minimal}: add cacert to nativeBuildInputs, silence warnings

[clacke]

We don't actually perform any network access, but since Racket 7.0,
when certain modules are loaded, they emit an annoying warning.
While compiling the bundled packages, this happens over and over and
pollutes the logs.

Having a correct SSL configuration prevents the warnings.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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