Add envvar to RDR authentication option

Given that the authentication key for RDR is something we'd like to keep
secret, I'm adding the ability to set this option through an environment
variable.

hoard/cli.py

@@ -41,7 +41,7 @@ def main():
     "source", type=click.Choice(["jpal", "llab", "whoas"], case_sensitive=False)
 )
 @click.argument("source_url")
-@click.option("--key", "-k", help="RDR authentication key.")
+@click.option("--key", "-k", envvar="HOARD_RDR_KEY", help="RDR authentication key.")
 @click.option(
     "--parent", "-p", default="root", help="Parent dataverse to ingest items into."
 )