Run gcc:tarball CI job for merge requests

Running gcc:tarball CI job for merge requests is consistent with how we
run gcc:out-of-tree CI job and should help identify problems with the
build system during the review process, not once merged during daily
runs. For the sake of time, unit and system tests associated with the
gcc:tarball CI job are excluded from merge requests.

.gitlab-ci.yml

@@ -810,7 +810,6 @@ gcc:tarball:
     RUN_MAKE_INSTALL: 1
   <<: *base_image
   <<: *build_job
-  <<: *schedules_tags_web_triggering_rules
   before_script:
     - tar --extract --file bind-*.tar.${TARBALL_EXTENSION}
     - rm -f bind-*.tar.${TARBALL_EXTENSION}