[Nix] Expose gtest for services tests

Summary: Allow for gtest to be found in develop environment

Test Plan:

nix develop
pkg-config --libs gtest
# should produce -L/nix/store/<hash>-gtest-1.11.0/lib -lgtest

Reviewers: atul, varun, max, karol

Reviewed By: varun, max

Subscribers: ashoat, tomek, adrian, abosh

Differential Revision: https://phab.comm.dev/D4781