[#188] Prepare Nix shell configuration for frontend and backend

Developed Nix shell configurations for both frontend and backend modules
to streamline the build and deployment processes and ensure alignment
with backend setup and dependencies. The new shell configurations
provide necessary tools and dependencies for development environments,
optimizing the overall development workflow.

Changes:
- Updated govtool/backend/default.nix to simplify package retrieval
  using `<nixpkgs>` and removed unnecessary sources file inclusion.
- Created govtool/backend/shell.nix to define additional dependencies
  like awscli, docker, git, and gnumake for the backend module.
- Added govtool/frontend/shell.nix to specify frontend module
  dependencies, ensuring consistency with the backend setup.

govtool/backend/default.nix

@@ -1,7 +1,4 @@
-# TODO: Remove the sources file and use the nixpkgs version provided from the
-# flakes lock file instead when the flakes feature is present and enabled in the
-# root of the project.
-{ pkgs ? (import ./sources.nix).pkgs }:
+{ pkgs ? import <nixpkgs> {} }:
 let
   # This is the version of the Haskell compiler we reccommend using.
   ghcPackages = pkgs.haskell.packages.ghc927;

govtool/backend/shell.nix

@@ -0,0 +1,16 @@
+{ pkgs ? import <nixpkgs> {} }:
+let
+  project = import ./default.nix { inherit pkgs; };
+in
+project.overrideAttrs (attrs: {
+  buildInputs = attrs.buildInputs ++ (with pkgs; [
+    awscli
+    docker
+    git
+    gnumake
+  ]);
+
+  shellHook = ''
+    ln -s ${project}/libexec/yarn-nix-example/node_modules node_modules
+  '';
+})

govtool/frontend/shell.nix

@@ -0,0 +1,12 @@
+{ pkgs ? import <nixpkgs> {} }:
+let
+  project = import ./default.nix { inherit pkgs; };
+in
+project.overrideAttrs (attrs: {
+  buildInputs = attrs.buildInputs ++ (with pkgs; [
+    awscli
+    docker
+    git
+    gnumake
+  ]);
+})