fix: Force bash for non-standard flags to exec

[jawnsy]

This addresses a regression introduced by #224. POSIX sh does not define arguments to the exec built-in, resulting in an error when using exec -a to set the process name (argv[0]).

This change executes /bin/bash instead of sh, and also changes to use exec -l, which automatically handles the hyphen prefix required to trigger login shell behavior.

This also explicitly runs /bin/sh, aligning the behavior of coder sh and the frontend Terminal application.

Tested this using Arch Linux (enterprise-dev image) as well as the base Debian image from DockerHub.

[coder@jawnsy-m coder-cli]$ go install ./cmd/coder
[coder@jawnsy-m coder-cli]$ which coder
/home/coder/go/bin/coder
[coder@jawnsy-m coder-cli]$ coder sh jawnsy-m
warning: version mismatch detected
  | Coder CLI version: unknown
  | Coder API version: 1.14.0+340-gd64a44cb3-20210123
  | 
  | tip: download the appropriate version here: https://github.com/cdr/coder-cli/releases
[coder@jawnsy-m ~]$ shopt login_shell
login_shell     on
[coder@jawnsy-m ~]$ exit
logout
[coder@jawnsy-m coder-cli]$ coder sh jawnsy-debian
warning: version mismatch detected
  | Coder CLI version: unknown
  | Coder API version: 1.14.0+340-gd64a44cb3-20210123
  | 
  | tip: download the appropriate version here: https://github.com/cdr/coder-cli/releases
root@jawnsy-debian:~# shopt login_shell
login_shell     on
root@jawnsy-debian:~# exit
logout

[shortcut-integration]

This pull request has been linked to Clubhouse Story #418: Use a login shell for terminals.