Changes

refactor(pipelines/pingcap/tidb): download componentsfrom OCI artifacts (#4077)

This pull request updates the CI pipelines for TiDB to use OCI-based
artifact downloads instead of the legacy file server, and improves
caching and test organization in the E2E test pipeline. The changes
modernize artifact management, streamline the preparation and testing
steps, and remove legacy environment variables.

**Migration to OCI-based artifact downloads:**
- Replaced usage of the `FILE_SERVER_URL` and legacy artifact fetching
with OCI artifact tags and the `download_pingcap_oci_artifact.sh` script
in both `ghpr_check2.groovy` and `pull_e2e_test.groovy` pipelines.
[[1]](diffhunk://#diff-2ec31602787b9f78109d406f68793c25b46975c1ccf60392c646f117bbd09085R49-R58)
[[2]](diffhunk://#diff-e81bb3e57b3459721e077248399744cdd97de52bb31e25eff35f68b031e9ebaaL39-R86)

**Environment and variable updates:**
- Removed the `FILE_SERVER_URL` environment variable and added
`OCI_ARTIFACT_HOST` to both pipelines for OCI registry access.
[[1]](diffhunk://#diff-2ec31602787b9f78109d406f68793c25b46975c1ccf60392c646f117bbd09085L27)
[[2]](diffhunk://#diff-e81bb3e57b3459721e077248399744cdd97de52bb31e25eff35f68b031e9ebaaL20-R22)
- Introduced computation of `OCI_TAG_PD` and `OCI_TAG_TIKV` for fetching
the correct component versions in `pull_e2e_test.groovy`.

**Pipeline and test improvements:**
- Improved caching strategy for binaries using more specific cache keys
in the E2E pipeline.
- Refactored test execution: split test stages into dedicated
directories, and updated artifact archiving logic for failed tests.

**Tests:**

-
https://do.pingcap.net/jenkins/job/pingcap/job/tidb/job/ghpr_check2/48557/
-
https://do.pingcap.net/jenkins/job/pingcap/job/tidb/job/pull_e2e_test/17/
 (commit: 4cb2349)