[desktop] add cross-env dependency and use it to set environment variables in desktop/package.json

Summary: windows ignores ENV=dev unless we use cross-env, which sets the environment variable in a windows-compatible way

Test Plan: when I console log process.env.ENV now, i see "dev" in my terminal. before this change, it was undefined

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12950