Nix feature should provide a way to access `flake#devShell`

[NickHu]

It is common for repositories which provide flakes to provide development shells under a top-level attribute flake#devShells, with flake#devShell being the default one (an alias for flake#devShells."<system>".default). These are accessed with the command nix develop flake#myShell; an excerpt from https://nixos.wiki/wiki/Flakes:

  # Used by `nix develop .#<name>`
  devShells."<system>"."<name>" = derivation;
  # Used by `nix develop`
  devShells."<system>".default = derivation;

However, you can't pass something like "flakeUri": "github:owner/repo#devShell" to the Nix feature provided here, because it (rightly) complains that this isn't a derivation (the correct derivation would be github:owner/repo#devShell.x86_64-linux). Fundamentally, this is because the implementation uses nix profile install, which searches for derivations under the flake#packages top-level attribute:

features/src/nix/post-install-steps.sh

Lines 11 to 15 in f7f05f7

	# Install Nix flake in profile if specified
	if [ ! -z "${FLAKEURI}" ] && [ "${FLAKEURI}" != "none" ]; then
	echo "Installing flake ${FLAKEURI} in profile..."
	nix profile install "${FLAKEURI}"
	fi

Ideally, somewhere in this feature a call should be made to nix develop to enable access to the development shells provided by a flake, rather than having to use the workaround of flake#devShell.x86_64-linux.

[NickHu]

cc @Chuxel