ci: pin the microbenchmarks code to a particular commit (#15374)

This change pins the checkout of the benchmarking-platform repo such
that updates to that repo do not affect this repo's CI without manual
intervention.

---------

Co-authored-by: brettlangdon <brett.langdon@datadoghq.com>

Author: emmettbutler

.gitlab/benchmarks/microbenchmarks.yml

@@ -10,6 +10,7 @@ variables:
   PACKAGE_IMAGE: registry.ddbuild.io/images/mirror/pypa/manylinux2014_x86_64:2025-04-12-5990e2d
   GITHUB_CLI_IMAGE: registry.ddbuild.io/images/dd-octo-sts-ci-base:2025.06-1
   BENCHMARKING_BRANCH: dd-trace-py
+  BENCHMARKING_COMMIT_SHA: e7bbac96e1ae9bfb5f8906dcdf103b08f5ca0805
 
 .benchmarks:
   stage: test
@@ -30,6 +31,7 @@ variables:
       git config --global url."https://gitlab-ci-token:${CI_JOB_TOKEN}@gitlab.ddbuild.io/DataDog/".insteadOf "https://github.com/DataDog/"
     fi
     git clone --branch "${BENCHMARKING_BRANCH}" https://github.com/DataDog/benchmarking-platform /platform
+    (cd /platform && git reset --hard "${BENCHMARKING_COMMIT_SHA}")
     export PATH="$PATH:/platform/steps"
 
     capture-hardware-software-info.sh