[Misc] Apply SonarCloud shelldre:S7689

SonarCloud rule fix

• 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

Other comments

No big issue, just a replacement within scripts