and you're just doing use flake in your .envrc?

what does your flake look like?

Yes

Right now like so

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
  };
  outputs = {nixpkgs, ...}: let
    system = "x86_64-linux";
    pkgs = import nixpkgs {
      inherit system;
      crossSystem = {
        config = "armv7l-linux-gnueabihf";
      };
    };
  in {
    devShells.${system}.default = import ./nix/shell.nix {inherit pkgs;};
  };
  nixConfig = {
    extraOptions = ''
      keep-outputs = true
      keep-derivations = true
    '';
  };
}

The shell.nix looks like so

{pkgs, ...}:
pkgs.mkShell {
  buildInputs = [
    pkgs.glibc
    pkgs.glibc.static
  ];

  makeFlags = [
    "CC=${pkgs.stdenv.cc.targetPrefix}cc"
    "CXX=${pkgs.stdenv.cc.targetPrefix}g++"
  ];
}

To be clear I only added the nixConfig bit today.

The rebuild happens once in a while I guess. E.g. yesterday evening I was using the dev shell (nix develop) but today in the morning gcc had to be rebuilt.

I haven't done any changes to my system or the flake in between those times

GC has this setup

    gc = {
      automatic = true;
      dates = "daily";
      options = "--delete-older-than 7d";
    };

Does this answer your questions?