sunbeam terraform apply fails behind proxy

I wanted to try the sunbeam setup for the first time, following [1]. It's a virtual machine behind a proxy, environment variables are set like this:

root@sunbeam:~# cat /etc/environment
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin"
HTTP_PROXY="http://<IP>:<PORT>"
HTTPS_PROXY="http://<IP>:<PORT>"
http_proxy="http://<IP>:<PORT>"
https_proxy="http://<IP>:<PORT>"
NO_PROXY="localhost, <our_domain>, 172.17.2.0/24"
no_proxy="localhost, <our_domain>, 172.17.2.0/24"

The initial steps (install openstack snap, prepare-node-script) seem to work with the proxy, but it fails at terraform apply:

---snip---
root@sunbeam:~# sudo sunbeam cluster bootstrap --accept-defaults
terraform apply failed:
Terraform used the selected providers to generate the following execution
plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # juju_application.sunbeam-machine will be created
  + resource "juju_application" "sunbeam-machine" {
      + constraints = (known after apply)
      + id = (known after apply)
      + model = "controller"
      + name = "sunbeam-machine"
      + placement = (known after apply)
      + principal = (known after apply)
      + trust = false
      + units = 0

      + charm {
          + channel = "latest/edge"
          + name = "sunbeam-machine"
          + revision = (known after apply)
          + series = "jammy"
        }
    }

Plan: 1 to add, 0 to change, 0 to destroy.
juju_application.sunbeam-machine: Creating...
juju_application.sunbeam-machine: Still creating... [10s elapsed]
juju_application.sunbeam-machine: Still creating... [20s elapsed]
juju_application.sunbeam-machine: Still creating... [30s elapsed]

Error: resolving with preferred channel: Post "https://api.charmhub.io/v2/charms/refresh": dial tcp 185.125.188.54:443: i/o timeout

  with juju_application.sunbeam-machine,
  on main.tf line 29, in resource "juju_application" "sunbeam-machine":
  29: resource "juju_application" "sunbeam-machine" {

Error: Command '['/snap/openstack/236/bin/terraform', 'apply', '-auto-approve', '-no-color', '-parallelism=1']' returned non-zero exit status 1.
---snip---

This error usually is related to a direct attempt to reach the URL instead of using the proxy. Is there something else missing to honor the proxy?

[1] https://microstack.run/#get-started