oh-my-posh: added option to supply config path

[yechielw]

Description

Checklist

• [ ] Change is backwards compatible.

• [ ] Code formatted with ./format.

• [ ] Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• [ ] Test cases updated/added. See example.

• [ ] Commit messages are formatted like

  {component}: {description}
  
  {long description}

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • [ ] Added myself as module maintainer. See example.

Maintainer CC