Upgrade setup-miniconda v2 to v3, fix arm64/Mambaforge/checkout ordering

[bschilder]

Summary

• Upgrade conda-incubator/setup-miniconda@v2 to @v3: v2 defaults architecture: x64, which downloads x86_64 installers on arm64 macOS runners (e.g. macos-15-arm64). v3 auto-detects the runner architecture.
• Note Mambaforge discontinuation: Mambaforge releases are no longer published by conda-forge, causing 404 errors when miniforge_variant: Mambaforge is used. Updated the input description to recommend Miniforge3.
• Move checkout before setup-miniconda: When users specify environment_file pointing to a repo file (e.g. inst/conda/env.yml), the file must exist before setup-miniconda runs. Previously checkout happened after, so environment_file could only work with URLs, not local repo paths.

Problem

When a downstream package uses:

miniforge_variant: Mambaforge
environment_file: inst/conda/my_env.yml

Two failures occur:

1. 404 on Mambaforge download — Mambaforge releases are discontinued
2. environment_file not found — checkout happens after setup-miniconda, so repo files don't exist yet

Even switching to Miniforge3, v2's x64 default means arm64 macOS runners install x86_64 conda via Rosetta instead of native arm64 builds.

Changes

File	Change
action.yml	setup-miniconda@v2 → @v3
action.yml	Move actions/checkout@v4 before setup-miniconda
action.yml	Update miniforge_variant description re: Mambaforge

Test plan

• Verify Miniforge3 installs correctly on macOS arm64 runners
• Verify environment_file with repo-local paths works after checkout reorder
• Verify conda env creation works on all 3 OS runners (ubuntu, macOS, windows)
• Verify no regressions for miniforge_variant: false (default — most users)