[nix] Update dev-shell.nix with comment on SystemConfiguration

Summary:
The reqwest requires the SystemConfiguration which is not available in the MacOS linker.

This adds a reminder comment to our nix shell to remove the SystemConfiguration dependency when the reqwest issue is resolved:
https://github.com/seanmonstar/reqwest/issues/2006#issue-1952682498

Test Plan: Successfully ran nix develop

Reviewers: ashoat, varun, O1 Nix

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10668