Fix error in Mac agent install script

[goodspark]

Fixes #14316

I also went ahead and changed all the single-character equality checks with double, since this script is meant to be run as a Bash script and the usage was inconsistent throughout the file.

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[bits-bot]

All committers have signed the CLA.

[mx-psi]

Thanks for the PR @goodspark! In order to accept it you need to sign the CLA by visiting https://cla.datadoghq.com/DataDog/datadog-agent?pullRequest=14317 and following the instructions there

[jrdemasi]

@mx-psi Since the bot indicated all committers signed the CLA can this get merged?

[KevinFairise2]

Hi
Thanks for the PR @goodspark, could you please rebase your PR on main branch so we can run all the tests before merging this?

[goodspark]

@KevinFairise2 done