Use `/usr/bin/env` for bash shebang

[abhillman]

Using env honors user preference if they override bash in $PATH or use a linux distribution that does not store bash at /bin/bash.

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References:

• https://www.oreilly.com/library/view/bash-cookbook/0596526784/ch15s01.html
• https://en.wikipedia.org/wiki/Shebang_(Unix)#Portability
• https://news.ycombinator.com/item?id=7596191

[abhillman]

cc/ @Augustin-Zidek