[keyserver] try to get identity config from file if env var is not present

Summary:
Depends on D10147

keep the env var logic in place for the Docker workflow, but now also try to get the JSON from a file for normal yarn dev workflow

Test Plan: tried both Docker and yarn dev workflows, saw the expected logs and confirmed that the socket address was the fake one i set in the config

Reviewers: bartek, ashoat

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10148