[services] Add hasEnvFlag

Summary:
Depends on D3956

https://linear.app/comm/issue/ENG-1104/consider-creating-hasenvflag-function

Test Plan:

cd services
yarn test-backup-service-dev-mode
yarn test-blob-service-dev-mode

and

cd services
git grep "std::getenv"

Should only print two results from lib/src/GlobalTools.cpp (also something from the tunnelbroker but it isn't relevant)

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3970