Disable oysteR security scanning for R flavors

[tomuben]

Background

OysteR stopped working without OSSIndex account, see https://github.com/exasol/script-languages-release/actions/runs/17948133968/job/51040027471?pr=1256.
Setting up credentials and passing them to the security scanner docker container is much effort.
For now, we will disable oysteR (and maybe focus on https://github.com/exasol/conda-vulnerability-scanner as a more generic approach).

Acceptance Criteria

• Disable oysteR security scanner on R flavors