Use echoconda's echoR_mini.yml for conda env on GHA

Point environment_file to echoconda's YAML via raw GitHub URL so
echotabix gets htslib/tabix/wget from the shared conda env definition.

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

Author: bschilder

.github/workflows/rworkflows.yml

@@ -52,3 +52,5 @@ 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