chore(ci): fix benchmarks images (#9181)

Currently microbenchmarks are broken, because wrong BASE_CI_IMAGE is
substituted in CI job.
This PR fixes references to CI images, so both macro- and
microbenchmarks work.

## Checklist

- [x] Change(s) are motivated and described in the PR description
- [x] Testing strategy is described if automated tests are not included
in the PR
- [x] Risks are described (performance impact, potential for breakage,
maintainability)
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed or label `changelog/no-changelog` is set
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/))
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))
- [x] If this PR changes the public interface, I've notified
`@DataDog/apm-tees`.

## Reviewer Checklist

- [x] Title is accurate
- [x] All changes are related to the pull request's stated goal
- [x] Description motivates each change
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- [x] Testing strategy adequately addresses listed risks
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] Release note makes sense to a user of the library
- [x] Author has acknowledged and discussed the performance implications
of this PR as reported in the benchmarks PR comment
- [x] 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)

.gitlab/benchmarks.yml

@@ -1,12 +1,12 @@
 variables:
-  BASE_CI_IMAGE: 486234852809.dkr.ecr.us-east-1.amazonaws.com/ci/benchmarking-platform:dd-trace-py
+  MICROBENCHMARKS_CI_IMAGE: 486234852809.dkr.ecr.us-east-1.amazonaws.com/ci/benchmarking-platform:dd-trace-py
   SLS_CI_IMAGE: registry.ddbuild.io/ci/serverless-tools:1
 
 .benchmarks:
   stage: benchmarks
   when: on_success
   tags: ["runner:apm-k8s-tweaked-metal"]
-  image: $BASE_CI_IMAGE
+  image: $MICROBENCHMARKS_CI_IMAGE
   interruptible: true
   timeout: 1h
   script:
@@ -37,7 +37,7 @@ benchmarks-pr-comment:
   stage: benchmarks-pr-comment
   when: on_success
   tags: ["arch:amd64"]
-  image: $BASE_CI_IMAGE
+  image: $MICROBENCHMARKS_CI_IMAGE
   script:
     - export REPORTS_DIR="$(pwd)/reports/" && (mkdir "${REPORTS_DIR}" || :)
     - git config --global url."https://gitlab-ci-token:${CI_JOB_TOKEN}@gitlab.ddbuild.io/DataDog/".insteadOf "https://github.com/DataDog/"

.gitlab/macrobenchmarks.yml

@@ -1,5 +1,5 @@
 variables:
-  BASE_CI_IMAGE: 486234852809.dkr.ecr.us-east-1.amazonaws.com/ci/benchmarking-platform:dd-trace-py-macrobenchmarks
+  MACROBENCHMARKS_CI_IMAGE: 486234852809.dkr.ecr.us-east-1.amazonaws.com/ci/benchmarking-platform:dd-trace-py-macrobenchmarks
 
 .macrobenchmarks:
   stage: macrobenchmarks
@@ -14,7 +14,7 @@ variables:
     # - if: $CI_COMMIT_REF_NAME == "main"
     #   when: always
   # If you have a problem with Gitlab cache, see Troubleshooting section in Benchmarking Platform docs
-  image: $BASE_CI_IMAGE
+  image: $MACROBENCHMARKS_CI_IMAGE
   script: |
     git clone --branch python/macrobenchmarks https://gitlab-ci-token:${CI_JOB_TOKEN}@gitlab.ddbuild.io/DataDog/benchmarking-platform platform && cd platform
     if [ "$BP_PYTHON_SCENARIO_DIR" == "flask-realworld" ]; then