gitlab-ci: disable perf tesing in scheduled runs

Release branches should be regularly run using gitlab-ci pipeline
schedules:
  https://gitlab.com/tarantool/tarantool/pipeline_schedules
It will help to detect flaky issues. But there is no need to rerun
too long running performance testing, to block it in schedules the
option 'schedules' in 'except:' field was set.

Part of #4974

.gitlab-ci.yml

@@ -32,6 +32,8 @@ variables:
   only:
     - master
     - /^.*-perf$/
+  except:
+    - schedules
   variables: &perf_vars_definition
     IMAGE_PERF: "${CI_REGISTRY}/${CI_PROJECT_PATH}/perf/ubuntu-bionic:perf_master"
     IMAGE_PERF_BUILT: "${CI_REGISTRY}/${CI_PROJECT_PATH}/perf_tmp/ubuntu-bionic:perf_${CI_COMMIT_SHORT_SHA}"