Installing on a machine behind a proxy

i ran into a few issue i had installing this project when run from behind a proxy:

the machine needed the following proxy settings:
```shell
sudo snap set system proxy.http="${HTTP_PROXY}"
sudo snap set system proxy.https="${HTTPS_PROXY}"
sudo lxc config set core.proxy_http="${HTTP_PROXY}"
sudo lxc config set core.proxy_https="${HTTPS_PROXY}"
git config --global http.proxy "${HTTP_PROXY}"
git config --global https.proxy "${HTTPS_PROXY}"
```

the script needed to be run like this:
```shell
http_proxy=${HTTP_PROXY} https_proxy=${HTTPS_PROXY} maas-dev-setup/setup-dev-env.sh --ok
```

and even `setup-region-via-ssh.bash` failed eventually when it got to setting-up the maas-dev container. I'm not sure which, but one of [these](https://github.com/maas/maas-dev-setup/blob/main/setup-region-via-ssh.bash#L61-L62) required:

```shell
http_proxy=${HTTP_PROXY} https_proxy=${HTTPS_PROXY} make
http_proxy=${HTTP_PROXY}} https_proxy=${HTTPS_PROXY} make snap-tree-sync
```

because without it -- it could not run the pip setup environment and likely the rest of the build process within them.