Add `RBS_COLLECTION_FILE` environment variable

[soutaro]

I would like to change the RBS library to read RBS collection configuration at RBS_COLLECTION_FILE environment variable, like BUNDLE_GEMFILE in Bundler.

This makes switching collection configurations easier.

# Read Gemfile.lock and rbs_collection.yaml
$ bundle exec rbs collection update

# Read Gemfile.steep and rbs_collection.steep.yaml
$ BUNDLE_GEMFILE=Gemfile.steep RBS_COLLECTION_FILE=rbs_collection.steep.yaml bundle exec rbs collection update

• --collection command line option has higher precedence than RBS_COLLECTION_FILE env var