util, refactor: Use GetPathArg to read "-rpccookiefile" value

Summary:
This is a partial backport of core#24675
https://github.com/bitcoin/bitcoin/pull/24675/commits/138c668e2b4d64279ddefbe07c1d9b7c3d3c537c

Depends on D16199

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16200