Fix shellcheck warnings in selenium component scripts (backport #14860)

[mergify]

Proposed Changes

This PR fixes the most critical shellcheck issues in the selenium test suite's component scripts (bin/components/*) without adding excessive quoting that changes the existing coding style.

Changes made:

• Added missing shebangs (#!/usr/bin/env bash) to 3 files that were missing them
• Replaced legacy backtick syntax (`command`) with modern $() syntax in 2 files
• Removed unused SCRIPT variable in rabbitmq component

What was NOT changed:

• Variable quoting: Remaining SC2086 warnings about unquoted variables are informational and left as-is per project coding style preference
• Array handling: Kept existing array syntax with command substitution
• Export statements: Not added as they would change behavior

These minimal changes improve script portability (shebangs) and maintainability (modern syntax) while respecting the project's existing coding conventions.

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

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• Mandatory: I (or my employer/client) have have signed the CA (see https://github.com/rabbitmq/cla)
• I have read the CONTRIBUTING.md document
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

This is a minimal, conservative approach that addresses only the most important shellcheck findings:

1. Missing shebangs: Critical for script portability and proper shell selection
2. Backticks to $(): Modern, more readable syntax that's easier to nest
3. Unused variables: Simple cleanup with no functional impact

The remaining shellcheck warnings (SC2086 about unquoted variables) are informational and don't represent actual bugs in this codebase. Adding quotes everywhere would be a larger stylistic change that goes beyond fixing actual issues.

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This is an automatic backport of pull request #14860 done by Mergify.