[CI] Fix pipeline validation check for CUSTOM_EOS_VERSION variable

Description

• Fix failing pipeline variable check for CUSTOM_EOS_VERSION

• Trigger pipelines now succeed:

  • Against main: https://gitlab.cern.ch/cta/CTA/-/pipelines/12765057
  • Against version: https://gitlab.cern.ch/cta/CTA/-/pipelines/12765147

• MR in EOS repo to include the platform in the variable.

Checklist

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

References

Closes #1278 (closed)