ci-tron: ensure the test jobs start with a clean job folder

By using `GIT_STRATEGY: empty` instead of `none`.

As documented in https://docs.gitlab.com/ci/runners/configure_runners/#git-strategy

    Unlike `none`, the `empty` Git strategy deletes and then re-creates
    a dedicated build directory before downloading cache or artifact files.
    With this strategy, the GitLab Runner hook scripts are still run (if
    provided) to allow for further behavior customization. Use the `empty`
    Git strategy when:
      - You do not need the repository data to be present.
      - You want a clean, controlled, or customized starting state every
        time a job runs.

In CI-tron jobs we put things there at the start of the job that we want
to keep through the start of the test and until the end of the job.

This requires gitlab-runner >= 17.10, which is already satisfied by all
ci-tron deployments.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35740>

.gitlab-ci/ci-tron/gitlab-ci.yml

@@ -3,7 +3,7 @@
   extends:
     - .ci-tron-b2c-job-v1
   variables:
-    GIT_STRATEGY: none
+    GIT_STRATEGY: empty
     B2C_VERSION: v0.9.17  # Linux 6.16.3
 
     CI_TRON_PATTERN__JOB_SUCCESS__REGEX: 'hwci: mesa: exit_code: 0\r$'