Migrate to rattler-build with staged outputs ("Megabuild")

[cbourjau]

This PR migrates this feedstock to rattler-build. It takes advantage of rattler-build's newly stabilized cached output feature to reuse build artifacts for different Python versions. To make this work, we also have to patch CMakeCache.txt (same approach taken in the PyTorch feedstock).

Other notable changes:

• nushell to DRY up Windows and Unix builds
• osx_arm64 is built natively (much faster and tested)

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• 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.

[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, recipe/recipe.yaml) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• ℹ️ Found bld.bat in recipe directory, but this is a recipe v1 (rattler-build recipe). rattler-build uses build.bat instead of bld.bat for Windows builds. Consider renaming bld.bat to build.bat.

For recipe/recipe.yaml:

• ℹ️ Found bld.bat in recipe directory, but this is a recipe v1 (rattler-build recipe). rattler-build uses build.bat instead of bld.bat for Windows builds. Consider renaming bld.bat to build.bat.

_{This message was generated by GitHub Actions workflow run https://github.com/conda-forge/conda-forge-webservices/actions/runs/22922418157. Examine the logs at this URL for more detail.}

[cbourjau]

https://github.com/prefix-dev/rattler-build/releases/tag/v0.63.0 landed support for staging between steps.

[conda-forge-admin]

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

I wanted to let you know that I linted all conda-recipes in your PR (recipe/recipe.yaml) and found some lint.

Here's what I've got...

For recipe/recipe.yaml:

• ❌ In conda-forge.yml: $.github_actions = {'self_hosted': True, 'timeout_minutes': 720, 'triggers': ['push', 'pull_request']}.

  {'self_hosted': True, 'timeout_minutes': 720, 'triggers': ['push', 'pull_request']} is not valid under any of the given schemas

  Schema

  {
    "anyOf": [
      {
        "$ref": "#/$defs/GithubActionsConfig"
      },
      {
        "type": "null"
      }
    ]
  }

_{This message was generated by GitHub Actions workflow run https://github.com/conda-forge/conda-forge-webservices/actions/runs/25058584535. Examine the logs at this URL for more detail.}

[cbourjau]

Windows error may be a variant of this upstream issue: prefix-dev/rattler-build#2444

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