Move capnproto to build reqs for cross-compiling

[jdblischak]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

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• Follow up to add osx_arm64 #42
• Support for openssl 3 was added in Rebuild for openssl3 #32
• xref: add osx_arm64 #42 (comment)

[conda-forge-admin]

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/meta.yaml) and found it was in an excellent condition.

[jakirkham]

Instead of doing this, would it be possible to build capnproto for the build_platform first? Then if build_platform != target_platform, build capnproto again for the target_platform?

This way we can relax the dependency on capnproto and handle the cross-compilation needs

[jdblischak]

Instead of doing this, would it be possible to build capnproto for the build_platform first? Then if build_platform != target_platform, build capnproto again for the target_platform?

I was just following the TODO in build.sh since I wanted to get rid of the manual mamba install step from the build script. I'll let the feedstock maintainers comment on the suitability/feasibility of your proposal.