[Nix] Patch aws-sdk-cpp to automatically find Aws.h

Summary:
Avoid having to explicitly pass
-DAWS_CORE_HEADER_FILE=$AWS_CORE_HEADER_FILE
to cmake.

Test Plan: N/A, later tunnelbroker build should suffice

Reviewers: varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4629