Use portable shebang for bash scripts (backport #542)

[mergify]

🦟 Bug fix

Summary

Replace #!/bin/bash with portable #!/usr/bin/env bash in all bash scripts for portability across systems where bash may not be installed at /bin/bash (e.g. NixOS).

This came up while packaging gazebo for nixpkgs, where builds fail because /bin/bash does not exist.

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

Generated-by: Claude Code

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This is an automatic backport of pull request #542 done by Mergify.