[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:
seanmonstar/reqwest#2006 (comment)

Test Plan: Successfully ran `nix develop`

Reviewers: ashoat, varun, O1 Nix

Reviewed By: ashoat

Subscribers: tomek

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

nix/dev-shell.nix

@@ -111,6 +111,7 @@ mkShell {
     CoreFoundation
     CoreServices
     Security
+    # required until https://github.com/seanmonstar/reqwest/issues/2006 is resolved
     SystemConfiguration
     libiconv  # identity service
   ]);