fix: remove nested variable in rules:if

Presentation

Fixes #151 (closed)

Used by TBC_DEFAULT_DOCKER_BUILD_TOOL See gitlab-org/gitlab#327780

Checklist

• Documented:
  • README.md reflects any job, variable or whichever visible change
  • kicker.json reflects any job, variable or whichever visible change
• Tested & examplified:
  • https://gitlab.com/clement.bois/sandbox/-/pipelines/2078673859 (Nothing set: buildah)
  • https://gitlab.com/clement.bois/sandbox/-/pipelines/2078677280 (TBC_DEFAULT_DOCKER_BUILD_TOOL: kaniko)
  • https://gitlab.com/clement.bois/sandbox/-/pipelines/2078678890 (DOCKER_BUILD_TOOL: kaniko)
  • Self-hosted tested too