In documentation https://docs.gitlab.com/ci/variables/predefined_variables/ :

CI_COMMIT_TAG Pre-pipeline The commit tag name. Available only in pipelines for tags.

Also https://docs.gitlab.com/ci/yaml/#rulesif

So, it seems to me this should do what you want.

rules:
  - if: $CI_COMMIT_TAG
    when: never
  - if: 'master' == $CI_COMMIT_BRANCH