feat: change `sh` to `bash`

[xxchan]

close #217

• no strong motivation to make it configurable yet
• bash's extensions are useful for scripting: arrays, loops, heredoc, etc.
• When the machine's sh is actually bash, using sh to execute will succeed without warnings. This caused a lot of trouble for me (i.e., why it works locally, but not in CI?)

[xxchan]

This is surprising 😅

https://stackoverflow.com/questions/8467424/echo-newline-in-bash-prints-literal-n