use NIX_SSL_CERT_FILE if set for vendoring cargo dependencies

[zickzackv]

fix issue #304483 when running fetchCargoTarball behind a cooperate MITM ssl proxy

Things done

• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[winterqt]

Arguably moot now because of #378288.

[zickzackv]

well, what is the other way of pulling rust dependencies then and do these methods work under a cooperate MITM ssl proxy? Shall I close this PR?

[zickzackv]

... and if the old code after merging #378288 still runs but with a warn message should that could at least support MITM SSL Proxies?