Allow custom path for global copilot-instructions.md

[lolodi]

Describe the feature or problem you'd like to solve

I'd like to have a way to customize the path for a global copilot-instructions.md so I can put it in my OneDrive and sync it across machines.

Proposed solution

Have an env variable set so I can configure the path or even better allow me to save it in my GitHub account so it syncs automatically across all my copilot sessions once I log in with my GitHub account.

Example prompts or workflows

Add to my instructions that I always want scripts to be measured for duration
Add to my instructions that interactive questions are encouraged to better define a plan
Add to my instructions file that I prefer PowerShell to be used as default language for scripting in windows.
You did an error here, this shouldn't have been done this way <.....>, please add this to my instructions file.

And all these instructions are synced across different machines automatically.

Additional context

Thinking more about it, the whole .copilot directory should be synced to my GitHub account or at least should be possible to add it to a cloud storage solution like OneDrive for easy sync. This way I can move and resume sessions across different boxes.