Proxy support seems to have regressed

There's an old closed issue #148 where someone reported problems installing dugite behind a proxy. The recommended solution was to set `HTTP_PROXY`/`HTTPS_PROXY` which is also documented in the [environment variable docs](https://github.com/desktop/dugite/blob/main/docs/environment-variables.md#installation). It seems, however, that when dugite migrated away from `requests`, those environment variables stopped having any effect.

Now that dugite is using the raw `https` module, it looks like the easiest solution might be to add `global-agent` as a dependency and use it inside `download-git.js`. Here's [how `@electron/get` uses `global-agent`](https://github.com/electron/get/blob/c99de4bd1f8c21e4da98a4b8ac37012530e99c50/src/proxy.ts), for example.

If that solution makes sense (or if you can suggest another solution you'd prefer), I'd be happy to submit a PR.

Start with download-git.js and the installation section of docs/environment-variables.md, then compare the current raw https usage with the linked @electron/get proxy.ts example. Confirm how HTTP_PROXY and HTTPS_PROXY are expected to affect downloads and define done as restoring proxy support without changing the documented environment-variable behavior.

Written by the indexing model from the issue text.