[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