ci: fix alpine sdist test [backport 3.16] (#14968)

Backport e1b6efa from #14963 to 3.16.

## Description

theacodes/cmarkgfm#84

`cmarkgfm` released a new version which no longer properly installs.
This explicitly installs a known working version.

## Testing

<!-- Describe your testing strategy or note what tests are included -->

## Risks

<!-- Note any risks associated with this change, or "None" if no risks
-->

## Additional Notes

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

Author: dd-octo-sts[bot]

.github/workflows/build_deploy.yml

@@ -116,8 +116,9 @@ jobs:
         run: apk add git gcc g++ musl-dev libffi-dev openssl-dev bash rust cargo make cmake patchelf
 
       - name: Check source package
+        # https://github.com/theacodes/cmarkgfm/issues/84
         run: |
-          pip install twine readme_renderer[md]
+          pip install twine readme_renderer[md] "cmarkgfm<2025.10.20"
           twine check dist/*.tar.gz
       - name: Install source package
         env: