Fix GHA: Mambaforge discontinued, switch to Miniforge3

Mambaforge releases are no longer published by conda-forge, causing
404 on download. Miniforge3 includes mamba by default since conda
23.10+ uses conda-libmamba-solver.

Note: rworkflows hardcodes setup-miniconda@v2 which defaults to x64
architecture. On arm64 macOS runners this installs x86_64 conda via
Rosetta. Proper fix requires rworkflows to upgrade to @V3.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bschilder

.github/workflows/rworkflows.yml

@@ -55,7 +55,7 @@ jobs:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run_docker: ${{ true }}
         docker_registry: ghcr.io
-        miniforge_variant: Mambaforge
+        miniforge_variant: Miniforge3
         miniforge_version: latest
         environment_file: inst/conda/echoR_mini.yml
         activate_environment: echoR_mini