Add opt-out SALSA_CI_DISABLE_GBP_SETUP_GITATTRIBUTES variable
It makes it possible to disable the gbp setup-gitattributes. Also, call gbp setup-gitattributes after gbp pull, to avoid creating staging and conflicting changes.
Document the use of the SALSA_CI_DISABLE_GBP_SETUP_GITATTRIBUTES variable in README.md