fix: use POSIX compatible comparison operator

[blucas]

What does this PR do?

Fixes issue on MacOS terminals (the default is zsh) when attempting to source the .envrc file.

The error is being generated for the syntax of the if statement

if [ "${KKA_AOA_EXCLUDE_ISTIO}" == "true" ]; then
  export KKA_AOA_EXCLUDE_KNATIVE="true"
else

for compatibility reasons, we should use = instead of ==

Related issues

Closes #88

Checklist before merging

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have checked the contributing document.
• I have checked the existing Pull Requests to see whether someone else has raised a similar idea or question.
• I have added tests that prove my fix is effective or that my feature works.