[Misc] Apply SonarCloud shelldre:S7689

Description

• Rule name: Command substitution should use modern "$()" syntax instead of backticks
• Rule code: shelldre:S7689
• Rule link: https://sonarcloud.io/project/issues?impactSeverities=LOW&rules=shelldre%3AS7689&issueStatuses=OPEN%2CCONFIRMED&id=cern-cta_CTA

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See our contributing docs

References

Closes #1387 (closed)