Update jenkins-agent.sh to work on ARM Macs

[cailafinn]

Description of work

Summary of work

The new arm macs don't work with the legacy versions of the jenkins agent jar file that the old x86 macs rely upon. Modify the script to take this into account.

Purpose of work

Allows the new arm macs to make use of the same script as the existing ones, keeping our deployment system as similar as possible.

There is no associated issue.

To test:

1. Check that the script runs and connects on both an old x86 runner and on a new ARM runner.

This does not require release notes because no user-facing changes have occurred.

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[MialLewis]

Looks good, couple of stylistic comments.

[MialLewis]

LGTM.

Haven't tested the changes, but as this is not part of the workbench package I don't think this is necessary.