CI: drop /tmp/artifacts upload to Gitlab

Contents of this folder are uploaded to S3 with shutzbot/upload_artifacts.sh
Also fixing epel installation in the mentioned script.

test/cases/diff-manifests.sh

@@ -2,9 +2,6 @@
 
 set -euo pipefail
 
-artifacts="/tmp/artifacts"
-mkdir -p "${artifacts}"
-
 # Colorful output.
 function greenprint {
     echo -e "\033[1;32m[$(date -Isecond)] ${1}\033[0m"
@@ -90,10 +87,10 @@ if (( err == 0 )); then
 fi
 
 greenprint "Manifests differ"
-echo "${diff}" > "${artifacts}/manifests.diff"
+echo "${diff}" > "manifests.diff"
 greenprint "Saved diff in job artifacts"
 
-artifacts_url="${CI_JOB_URL}/artifacts/browse/ci-artifacts"
+artifacts_url="${CI_JOB_URL}/artifacts/browse"
 
 cat > "${review_data_file}" << EOF
 {"body":"⚠️ This PR introduces changes in at least one manifest (when comparing PR HEAD ${head} with the ${basebranch} merge-base ${mergebase}).  Please review the changes.  The changes can be found in the [artifacts of the \`Manifest-diff\` job [0]](${artifacts_url}) as \`manifests.diff\`.\n\n${merge_base_fail}[0] ${artifacts_url}","event":"COMMENT"}