Add artifacts for SLO checks to enable notifications (#8184)

## Summary of changes

Add SLO check artifacts to the SLO check jobs to enable notifications.

As a reminder:
1. We are not blocking on SLO breaches (we have `allow_failure: true`);
2. Notifications will only trigger on big regressions, since SLOs were
set with a wide slack from benchmarking measurements.

## Reason for change

Necessary to enable notification jobs.

## Implementation details

## Test coverage

## Other details
<!-- Fixes #{issue} -->


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Author: igoragoli

.gitlab/benchmarks/macrobenchmarks.yml

@@ -743,6 +743,12 @@ check-slo-breaches:
     DDOCTOSTS_POLICY: "gitlab.github-access.read-contents"
     ARTIFACTS_DIR: "reports"
     SLO_FILE: ".gitlab/benchmarks/bp-runner.fail-on-breach.yml"
+  artifacts:
+    name: "artifacts"
+    when: always
+    paths:
+      - reports/
+    expire_in: 3 months
 
 notify-slo-breaches:
   extends: .notify-slo-breaches