Revert "Nix: move to devshell"

This reverts commit 8c9f075a4e.

.envrc

@@ -1,12 +1 @@
-#!/usr/bin/env bash
-# ^ added for shellcheck and file-type detection
-
-# Watch & reload direnv on change
-watch_file devshell.toml
-
-if [[ $(type -t use_flake) != function ]]; then
-  echo "ERROR: use_flake function missing."
-  echo "Please update direnv to v2.30.0 or later."
-  exit 1
-fi
-use flake
+use flake