Add SALSA_CI_DISABLE_EXPORTORIG_FALLBACK option
In this commit, allow to disable gbp export-orig fallback.
Even though maintainer forgot to push pristine-tar branch, CI continues to process with origtargz fallback. It may be harmful because it can be overlooked with this behavior.
By default, SALSA_CI_DISABLE_EXPORTORIG_FALLBACK=0 as same as before to keep compatibility.
Signed-off-by: Kentaro Hayashi kenhys@gmail.com