Improve build scripts for bash compatibility and update README

[Septa2112]

Issue

Now running ./build_llvm.sh will cause an issue: ./build_llvm.sh: 17: source: not found.

Because the script is executed with the system default shell (sh), which does not support the source command used in the script, it will cause the Python virtual environment not to activate actually.

How to Fix

The script should either be explicitly run using bash (e.g., bash ./build_llvm.sh) or the script itself should specify bash as the default shell by adding the shebang line #!/usr/bin/env bash at the top.

My improvements:

• Updated the README.
• Modified the shebang line.
• Added set -e to make the script exit immediately if any command fails.

[lum1n0us]

There is no doubt that this can solve the problem. However, #!/bin/bash is commonly used in bash scripts within this project. It is difficult not to consider whether we should apply this modification to all other bash scripts to prevent similar default shell issues.

Personally, I believe the best approach is to verify the default shell and set it to bash when preparing the environment.

## identify available shells.
$ cat /etc/shells

## verify current default shell
$ echo "$SHELL"

## change the default shell.
$ chsh -s /path/to/new/shell

[yamt]

@Septa2112 does #4541 fix the issue for your env?

[Septa2112]

@yamt yes, it works for my env. And your changes are cleaner and more reasonable. I'm considering closing my PR and adopting yours.

[yamt]

@yamt yes, it works for my env. And your changes are cleaner and more reasonable. I'm considering closing my PR and adopting yours.

thank you for confirmation.