feat: enable proxy via reading envrionment variables

[Erven2016]

Allow user to apply proxy by overriding proxy-related environment variables.

Related issue: #164

Ref:

Apparently, you can set the proxy by using a --proxy-server option on the command-line (e.g. --proxy-server=socks5://127.0.0.1:9050)

Originally posted by @uhoreg in element-hq/element-desktop#772 (comment)

[flathubbot]

Started test build 159888

[flathubbot]

Build 159888 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/142968/im.riot.Riot.flatpakref

[SISheogorath]

bot, build

[flathubbot]

Queued test build for im.riot.Riot.

[flathubbot]

Started test build 160663

[flathubbot]

Build 160663 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/143737/im.riot.Riot.flatpakref

[SISheogorath]

Thanks a lot! That should make things easier for people :)