fix: variable reference syntax

[connorhaugh]

Configuration Pull Request

Per: https://stackoverflow.com/questions/4181703/how-to-concatenate-string-variables-in-bash
We need the ${} to reference variables and avoid ambiguity

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Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.
• Think about how this change will affect Open edX operators and update the wiki page for the next Open edX release if needed

[schenedx]

👍