Update bash-completion sourcing.
Update bash-completion sourcing.

The previous sourcing is replaced with what the current “caveats” specify (see `brew info bash-completion@2`).

And, thanks to Homebrew/homebrew-core#36254, any bash-completions meant for bash-completion v1 are now included, thanks to the environment variable.

Dreamseer/dotfiles,Dreamseer/dotfiles,Dreamseer/dotfiles