Remove environment_file URL (setup-miniconda only supports local paths)

Just install Miniforge3 so conda binary is available on PATH.
echoconda handles env creation at runtime.

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

Author: bschilder

.github/workflows/rworkflows.yml

@@ -52,5 +52,3 @@ jobs:
         docker_registry: ghcr.io
         miniforge_variant: Miniforge3
         miniforge_version: latest
-        environment_file: https://raw.githubusercontent.com/RajLabMSSM/echoconda/main/inst/conda/echoR_mini.yml
-        activate_environment: echoR_mini