ci: Fix for GitLab 18.2.2 upgrade

Now the `subject` from S3 id_token JWT refers to the target project when
we are in an MR pipeline, due to this patch [1].
So, let's use workflow variables to help us define where the S3 paths
should be pointing at.

[1] 7e9f329e8f

Signed-off-by: Guilherme Gallo <guilherme.gallo@collabora.com>
Suggested-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36832>

.gitlab-ci.yml

@@ -45,6 +45,7 @@ workflow:
         JOB_PRIORITY: 75
         # fast-fail in merge pipelines: stop early if we get this many unexpected fails/crashes
         DEQP_RUNNER_MAX_FAILS: 40
+        S3_PROJECT_PATH: ${CI_MERGE_REQUEST_SOURCE_PROJECT_PATH}
     # Post-merge pipeline
     - if: &is-post-merge $GITLAB_USER_LOGIN == "marge-bot" && $CI_PIPELINE_SOURCE == "push"
       variables:
@@ -53,6 +54,8 @@ workflow:
         FDO_RUNNER_JOB_PRIORITY_TAG_AARCH64: priority:high-aarch64
     # Pre-merge pipeline (because merge pipelines are already caught above)
     - if: &is-merge-request $CI_PIPELINE_SOURCE == "merge_request_event"
+      variables:
+        S3_PROJECT_PATH: ${CI_MERGE_REQUEST_SOURCE_PROJECT_PATH}
     # Push to a branch on a fork
     - if: &is-push-to-fork $CI_PROJECT_NAMESPACE != "mesa" && $CI_PIPELINE_SOURCE == "push"
     # Nightly pipeline
@@ -104,8 +107,11 @@ variables:
   S3_TRACIE_PRIVATE_BUCKET: mesa-tracie-private
   # Base path used for various artifacts
   S3_BASE_PATH: "${S3_HOST}/${S3_KERNEL_BUCKET}"
+  # Since GitLab 18.2.2, the JWT subject for MRs are the target project path
+  # This can be overridden by the variables in the workflow rules
+  S3_PROJECT_PATH: ${CI_PROJECT_PATH}
   # per-pipeline artifact storage on MinIO
-  PIPELINE_ARTIFACTS_BASE: ${S3_HOST}/${S3_ARTIFACTS_BUCKET}/${CI_PROJECT_PATH}/${CI_PIPELINE_ID}
+  PIPELINE_ARTIFACTS_BASE: ${S3_HOST}/${S3_ARTIFACTS_BUCKET}/${S3_PROJECT_PATH}/${CI_PIPELINE_ID}
   # per-job artifact storage on MinIO
   JOB_ARTIFACTS_BASE: ${PIPELINE_ARTIFACTS_BASE}/${CI_JOB_ID}
   # reference images stored for traces