Fixes NixOS/nixpkgs#370494 and #108.
Long-ass explanation is here: NixOS/nixpkgs#370494 (comment).

When running under nix environment _FORTIFY_SOURCE=3 is set by default.
Currently the CFLAGS are passed down to the underlying jemalloc compiler.

If we're running cargo build, it means that something along the lines:
-O0 -ffunction-sections -fdata-sections -fPIC -m64 -Wall is passed down (optimisation disabled, because debug mode).
Because optimisations are disabled, we're getting errors:

In file included from /nix/store/hn4s3cq7368jkb2db02gzxhdzfa3g9zp-glibc-2.40-36-dev/include/errno.h:25,
                 from conftest.c:101:
/nix/store/hn4s3cq7368jkb2db02gzxhdzfa3g9zp-glibc-2.40-36-dev/include/features.h:422:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
  422 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
      |    ^~~~~~~

This simply removes passing down the CFLAGS, so they can be set automatically by underlying autoconfigures and not dependent on the build profile.
Plus jemalloc/jemalloc#1196 was solved long time ago, so I think the reason those were added is gone.

P.S this issue is reproducible by running the master branch, flake.nix in the root repo

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    rust-overlay = {
      url = "github:oxalica/rust-overlay";
      inputs = {
        nixpkgs.follows = "nixpkgs";
      };
    };
  };
  outputs = { self, nixpkgs, flake-utils, rust-overlay }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        overlays = [ (import rust-overlay) ];
        pkgs = import nixpkgs {
          inherit system overlays;
        };
        rustToolchain = pkgs.pkgsBuildHost.rust-bin.stable.latest.default;
        buildInputs = with pkgs; [
          rustToolchain
        ];
      in
      with pkgs;
      {
        devShells.default = mkShell {
          inherit buildInputs;
        };
      }
    );
}

and running

$ git add flake.nix
$ nix develop
$ cargo -vv build