deps: fetch build artefacts using a proxy where necessary

[jnsgruk]

I've been struggling to build Signal from source where the build machine is behind a proxy.

This change will have no effect during build unless the HTTPS_PROXY variable is set, in which case it establishes an agent that will ensure the https.get calls use the proxy correctly.

I'm open to suggestions on a different approach if the addition of another dependency is too undesirable.

[jnsgruk]

@indutny-signal - sorry for the direct ping, you seem to be the most active contributor on this project. Any chance you could take a look here? :)

[jnsgruk]

@indutny-signal / @trevor-signal are you able to take a look at this with a view to getting it in a future Signal Desktop release? Thank you! :pray:

[jnsgruk]

@indutny-signal @trevor-signal @jamiebuilds-signal any chance of a look at this? Many thanks! :slightly_smiling_face:

[jnsgruk]

@indutny-signal @trevor-signal @jamiebuilds-signal any update on this at all? Thank you!

[jnsgruk]

@ayumi-signal is there any way to get some attention on this? :) thank you!