services/agent/agentlib: make credsDir absolute before passing to agent
services/agent/agentlib: make credsDir absolute before passing to agent

See https://github.com/vanadium/issues/issues/1283

Change-Id: I701f4f4af9cfc131c1c80bee64aefc465c9bc134

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref