Ability to add custom CA roots for downloads via env var

[passy]

This allows you to set a custom CA root for downloads, e.g. for transparent caching of downloads via a proxy.

Test Plan

env HTTP_PROXY=http://localhost:8080 HTTPS_PROXY=http://localhost:8080 NODE_EXTRA_CA_CERTS=~/.mitmproxy/mitmproxy-ca-cert.pem ./app-builder download --url="https://github.com/facebook/flipper/releases/download/v0.34.0/Flipper-mac.dmg" --output="/tmp/download"

Open Questions

I saw that this tries to reuse as much from npm/node as possible to make the integration with electron-builder etc. very convenient. I'm not sure if there's a more app-builder-specific env var as an alternative to that should be accepted here?

Is there any place where the env vars are currently documented and this should be mentioned?

[develar]

Is there any place where the env vars are currently documented and this should be mentioned?

No.

if there's a more app-builder-specific env var as an alternative to that should be accepted here?

If you use it for nodejs, NODE_EXTRA_CA_CERTS name is ok. If not, we can think about what prefix should be used to make name more generic.

[passy]

Thanks for merging. :)