Does

  nix develop -c jekyll build

not do the job for you? (For a single development command—for deploy 'nix build' should be set up to do the right thing, including setting LC_ALL if necessary; and to enter a dev shell just 'nix develop' should work. Nixpkgs installs glibc with a full complement of locales by default, which has its downsides size-wise but should prevent any troubles when setting LC_ALL=fr_FR.)

Or, for an automatic setup,

  cat > .envrc
  use flake
  ^D

or, for an automatic setup with caching[1],

  cat > .envrc
  if ! has nix_direnv_version || ! nix_direnv_version 2.2.0; then
    source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/2.2.0/direnvrc" "sha256-5EwyKnkJNQeXrRkYbwwRBcXbibosCJqyIUuz9Xq+LRc="
  fi
  use flake
  ^D

(I don't have nix-direnv installed systemwide because it ignores the --no-write-lock-file flag I need to pass sometimes. If you put some of your config in default.nix or shell.nix or whatever, you’ll need to watch_file resp. nix_direnv_watch_file those.)

[1] https://github.com/nix-community/nix-direnv#direnv-source_ur...