Pin boost-cpp directly

[xhochy]

We seem to also link some boost symbols into pyarrow. This should guarantee through the new run_exports pinning mechanism that we only see installs of arrow-cpp and pyarrow with the same boost-cpp version.

This sadly does not fix the issue with boost-cpp and libboost.

Checklist

• [x] Used a fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[xhochy]

@conda-forge-admin please update circle

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-webservice.

I just wanted to let you know that I updated the circle-ci deploy key and followed the project.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[kszucs]

Just tried to approve :)

[kszucs]

Thanks @xhochy !