dev: add en_GB locale archive to devenv

Provide a Nix glibc locale archive with en_GB.UTF-8 so tools inside the devenv shell inherit a valid UTF-8 locale.

devenv.nix

@@ -2,8 +2,18 @@
   pkgs,
   lib,
   ...
-}: {
+}: let
+  devLocales = pkgs.glibcLocales.override {
+    allLocales = false;
+    locales = [
+      "en_GB.UTF-8/UTF-8"
+      "en_US.UTF-8/UTF-8"
+    ];
+  };
+in {
   env = {
+    LANG = "en_GB.UTF-8";
+    LOCALE_ARCHIVE = "${devLocales}/lib/locale/locale-archive";
     MIX_OS_DEPS_COMPILE_PARTITION_COUNT = 8;
     NODE_ENV = "development";
     # Delay npm dependency resolution to reduce rushed supply-chain updates.