Use /bin/bash in shebang

[Daniel15]

Summary
Use /bin/bash as the shebang, as the script may use Bash-specific features.

Test plan
Run yarn with /bin/sh symlinked to /bin/dash and ensure it still works.

Closes #3321

[valpackett]

This is a terrible idea, /bin/bash is only available on GNU/Linux. This would break yarn on all of my machines, since they run FreeBSD. The real solution is to use [ instead of [[.

[Daniel15]

@bestander Do you know why this was changed from [ to [[?

[bestander]

It started complaining on my mac after I merged this #3245
No idea what I was doing there

[Daniel15]

@myfreeweb Do you know how to fix #3260 without using [[? I'm not very familiar with shell scripting.

[valpackett]

@Daniel15 use = instead of -eq. = is for strings, -eq is for numbers

[Daniel15]

Sounds good. Let's fix it the proper way.