feat(nix): nix and back again

I tried Nix, but it had too many downsides so I removed it.

1. Didn't like that all files are immutable and simple config changes
need a complete rebuild.
2. Setting up a new Mac didn't work as smoothly as promised. Not worth
the effort.
3. It sucked a lot to always have to type in the password twice on each
darwin-rebuild
4. It solves problems I never had.

bin/homebrew-backup

@@ -0,0 +1,2 @@
+#!/bin/bash
+brew bundle dump --force --describe --file=~/.dotfiles/homebrew/$(hostname)

bin/homebrew-restore

@@ -0,0 +1,3 @@
+#!/bin/bash
+brew bundle cleanup --force --file ~/.dotfiles/homebrew/$(hostname)
+brew bundle --file ~/.dotfiles/homebrew/$(hostname)

bin/update

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+# Ask for the administrator password upfront
+sudo -v
+
+# Keep-alive: update existing `sudo` time stamp until script has finished
+while true; do
+	sudo -n true
+	sleep 60
+	kill -0 "$$" || exit
+done 2>/dev/null &
+
+cd ~/.dotfiles
+
+# Update Homebrew
+brew update
+brew outdated
+brew upgrade
+brew cleanup
+
+# Ruby
+gem update --system
+gem update
+gem cleanup all
+
+# Update tmux plugins
+~/.tmux/plugins/tpm/bin/update_plugins all
+
+# Update GitHub extensions
+gh extension upgrade --all
+
+# Update Fish plugins
+fisher update
+
+# Update Neovim
+nvim --headless "+Lazy! sync" +qa
+
+# Update macOS software
+sudo softwareupdate -i -a