Config file path will change in the future

[kylewillmon]

The settings.yaml file will be moving in the next release of the CLI, which will break the method used here to set phylum_token.

Instead of modifying the settings.yaml file, this action should be setting an environment variable that can be used in future steps.

The CLI uses the PHYLUM_API_KEY environment variable (which has existed for a while, but is not yet documented until phylum-dev/cli#228 is merged), so setting this should make everything work in a way that will not break when the settings.yaml file moves.