text: Title: `robot:skip` and `robot:exclude` tags do not support variables
  Body: I try to use a variable in the second test so that the resulting tag is `robot:skip`:
  
  
  ```robotframework
  *** Variables ***
  ${SKIP_FLAG}    skip
  
  *** Test Cases ***
  Test One
      Log  Test One was executed.
      VAR  ${SKIP_FLAG}  skip  scope=suite
  
  Test two
      [Tags]  robot:${SKIP_FLAG}
      Log  Test Two was executed.
  ```
  My expectation was that I can conditionally skip other tests _completely_.
  The tag gets set correctly, but the test still gets executed. 
  
  https://robotframework.org/code/?codeProject=N4IgdghgtgpiBcIDCBXAzgFwPZRAGhABMY0BjAJwEsAHDSrMBEfEAM0oBsSEBtUdrgDlocRABUSGAMopKGGADpyWAEZYMLUg3lgNiADpgAVCYAEANQhUIKrmlMmjhgCTApAaQCSABQD6AMQAZAEEAcQBfUyjTNABrGkNDR1MJTFMkCDQSBxNDVIxTAHkwGENo00CsAHMo-KKS0wB3TNMYAA8YUhR5QgUy6PNggCUo1w8fAJCIqLiaGa1qGABeNFl5RLA6jEasfqieMQgqtABdKOU1DHgxrz8gsPC9iurayRSdppb2zu6YXpBwicCMQqAA3P7eZQAK06egw5BQMAIF3U5hg5DQ9EYiAA7AoAIwElgojDBcjHBDAcLImAAR1k5BgsF0aF4J3CQA