ci: skip serverless benchmark failures to unblock CI [backport 3.3] (#12804)

Backport 39806c6 from #12800 to 3.3.

Allow failures temporarily while we figure out what changed and is
broken.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Co-authored-by: Brett Langdon <[email protected]>

Author: github-actions[bot]

.gitlab/benchmarks/serverless.yml

@@ -7,6 +7,8 @@ benchmark-serverless:
     project: DataDog/serverless-tools
     strategy: depend
   needs: []
+  # TODO: Revert this once the build issue has been resolved
+  allow_failure: true
   variables:
     UPSTREAM_PIPELINE_ID: $CI_PIPELINE_ID
     UPSTREAM_PROJECT_URL: $CI_PROJECT_URL