refactor: add bash shebang for shellcheck

[mjgallag]

https://www.shellcheck.net/wiki/SC2148 (tips depend on target shell)
https://direnv.net/#how-it-works (.envrc is loaded into a bash sub-shell)

[mikeland73]

@mjgallag this LGTM!

One caveat is that this file is not replaced automatically (because it would require re-allow from direnv).

Folks that already have .envrc would need to do: devbox gen direnv --force and direnv allow

(We've talked about versioning this file in the past, but that's outside the scope of this change)

[gcurtis]

Not as familiar with the envrc stuff so this might not matter, but is the file generated from this template ever executed directly (e.g., ./envrc)? Although it's unlikely, that would fail if there's no bash in /bin/bash. Another option is to add a shellcheck directive with # shellcheck shell=bash.

[mjgallag]

@gcurtis direnv uses the bash path that is set at build time:
https://github.com/direnv/direnv/blob/ae5a61848a06da4251b91fd084bffa9a5732fb89/internal/cmd/rc.go#L271
https://github.com/NixOS/nixpkgs/blob/aaa7fb5840f278da8010fe7466c3d952f7e536da/pkgs/by-name/di/direnv/package.nix#L17-L19
https://github.com/Homebrew/homebrew-core/blob/fb3c5a4fcbd4d4974499ed9b46304d8d0881cbc3/Formula/d/direnv.rb#L24

I could envision maybe running . ./envrc or ./envrc directly as a convenience when testing changes to it, but the shebang shouldn't have any effect as far as direnv is concerned. That being said I don't have a strong preference here between #!/bin/bash, #!/usr/bin/env bash or # shellcheck shell=bash.