Remove bashisms from Bash shell implementation

What does this MR do?

Removes bashisms from the shell script generation mechanisms in bash.go.

Why was this MR needed?

Bashisms --- non-standard extensions of the POSIX shell language --- break POSIX shell support; i.e., containers lacking Bash were failing.

What's the best way to test this MR?

I have a reproducer: https://gitlab.com/realtime-neil/ubuntu-bashless

What are the relevant issue numbers?

#1170 (closed)

Closes #3622 (closed)

Closes #28026 (closed)