Improved flake support

• don't override $SHELL variable
• also output shellHook output when using cached nix shell
• fix usage if direnv_layout_dir points not to .direnv
• fallback to perl on macOS if realpath is not installed
• prevent garbage collection of flake inputs by creating a gcroot

Flake support

This release adds a use_flake implementation for nix flakes.

Code cleanups

This is hopefully the last release candidate.
It should not introduce any changes but only
internal code cleanups done.

Fix ssl

Previously invalid SSL certificates were exported: #33

Flake support

This release adds a use_flake function

Sufficient

Our first release!