Set DTC_PROJECT_BRANCH environment variable

Related issue: https://github.com/docToolchain/docToolchain/issues/950

Once the PR https://github.com/docToolchain/docToolchain/pull/951 is merged it makes sense to set the DTC_PROJECT_BRANCH_PATCHED in the CI pipeline.

• For GitHub, the value ${{ github.head_ref || github.ref_name }} was suggested here: https://stackoverflow.com/a/71158878
• For GitLab, the value $CI_COMMIT_REF_NAME seems to be what we need: https://docs.gitlab.com/ee/ci/variables/predefined_variables.html