fix: quote inputs and improve shell conditionals for safety

[Thedarkmatter10]

This PR improves safety and consistency of the GitHub Actions workflow (build-test-images.yml) file by:

• Using [[ ... ]] instead of [ ... ] for conditionals.
• Adding double quotes around inputs and refs to avoid evaluation issues.

This helps prevent bugs in shell parsing, especially with empty or misinterpreted input values.

@katiewasnothere @wlan0

[katiewasnothere]

Thanks for the fix! :) will merge as soon as the CI completes

[crosbymichael]

We just enabled signed commits as a requirement. Can you setup signing for your commit? I'm using SSH based signing and it is quick to get setup.

See Github's docs here: https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification#ssh-commit-signature-verification

[Thedarkmatter10]

I've signed the commit using SSH as requested. Please let me know if there's anything else to fix
@crosbymichael