fix: use `/usr/bin/env bash` instead of hardcoded bash path

[Quoteme]

This makes your extension work on distros like NixOS, which have different locations for where bash is installed.

[BerriJ]

I just added this package to the NixOS store, so the easiest (and a declarative) way to install this on NixOS is through your configuration. :)

On the PR itself: I suggest using /bin/sh for the shebang line. POSIX mandates a shell named sh, so every Unix-like system (including Linux) has a sh binary or a symlink to a compatible shell. This does not hold for bash.