Plugin manager doesn't respect system proxy settings

I’m behind a HTTPS proxy, and direct connections to port 443 are blocked by my firewall. I’ve got a proxy setting in System Preferences and other stuff works fine, but a direct connection will fail. However, Glyphs tries to make a direct connection:

This doesn’t work, so plugins can’t be installed.

The actual network access is handled by git. So I’m not sure I can do much about this.

git respects http_proxy and https_proxy environment variables. If you can get the appropriate values from the OS and set them in the environment it should work.