fix: append export AWS_ENDPOINT_URL_DEADLINE at end of worker SSM commands

What was the problem/requirement? (What/Why)

When we spin up multiple workers, the 2nd worker and so forth always fails since in the command to install the worker agent, we runuser as a deadline-worker user that has not been created yet, trying to export an env variable.

What was the solution? (How)

Move the runuser command to the end of the commands, so the user is actually created during the worker agent install.

What is the impact of this change?

The failing worker-agent tests that use a separate worker from the main session worker should pass again

How was this change tested?

Built into worker-agent repo, and used hatch run cross-os-e2e-test to verify on Linux that it works

Was this change documented?

Is this a breaking change?

No

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aws-deadline / deadline-cloud-test-fixtures