ci: update regression check rules for release tags (#13141)

Fix for #13135 which incorrectly was looking at the branch name and not
the tag name.

## Checklist
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- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
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[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
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- Title is accurate
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[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
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implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: brettlangdon

.gitlab/benchmarks/microbenchmarks.yml

@@ -184,8 +184,12 @@ benchmarks-pr-comment:
 check-big-regressions:
   stage: report
   rules:
-    # Run and warn on release branches, but don't fail the pipeline
-    - if: $CI_COMMIT_BRANCH =~ /^[0-9]+\.[0-9]+$/
+    # Run and warn on releases, but don't fail the pipeline
+    #   v2.10.0
+    #   v2.10.1
+    #   v2.10.0rc0
+    #   v2.10.0rc5
+    - if: $CI_COMMIT_TAG =~ /^v[0-9]+\.[0-9]+\.[0-9]+(rc[0-9]+)?$/
       allow_failure: true
     - allow_failure: false
   when: always