chore(ci): new testrunner for check_new_flaky_tests [2.17] (#11947)

## 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)

---------

Signed-off-by: Juanjo Alvarez <juanjo.alvarezmartinez@datadoghq.com>
Co-authored-by: Brett Langdon <brett.langdon@datadoghq.com>

.gitlab-ci.yml

@@ -89,6 +89,7 @@ deploy_to_di_backend:manual:
 check_new_flaky_tests:
   stage: quality-gate
   extends: .testrunner
+  image: registry.ddbuild.io/images/mirror/dd-trace-py/testrunner:0a50e839f4b1600f02157518b8d016451b346578@sha256:5dae9bc7872f69b31b612690f0748c7ad71ab90ef28a754b2ae93d0ba505837b
   script:
     - export DD_SITE=datadoghq.com
     - export DD_API_KEY=$(aws ssm get-parameter --region us-east-1 --name ci.${CI_PROJECT_NAME}.dd-api-key-qualitygate --with-decryption --query "Parameter.Value" --out text)

.gitlab/testrunner.yml

@@ -4,5 +4,5 @@
   tags: [ "arch:amd64" ]
   timeout: 20m
   before_script:
-    - pyenv global 3.12 3.7 3.8 3.9 3.10 3.11 3.13-dev
+    - pyenv global 3.12 3.7 3.8 3.9 3.10 3.11
     - export _CI_DD_AGENT_URL=http://${HOST_IP}:8126/

.gitlab/tests.yml

@@ -13,7 +13,7 @@ variables:
   tags: [ "arch:amd64" ]
   timeout: 20m
   before_script:
-    - pyenv global 3.12 3.7 3.8 3.9 3.10 3.11 3.13-dev
+    - pyenv global 3.12 3.7 3.8 3.9 3.10 3.11
     - export _CI_DD_AGENT_URL=http://${HOST_IP}:8126/