unable to get repo-key in proxy-only environment

[Daniel Krambrock]

In ubuntu 22.04 autoinstall I'm trying to add an extra apt-repo with:

```
  apt:
    sources:
      reponame:
        source: "deb http://apt.puppetlabs.com $RELEASE puppet7"
        keyid: D681 1ED3 ADEE B844 1AF5 AA8F 4528 B6CD 9E61 EF26
```

curtin is trying to get the key from keyerver.ubuntu.com with gpg:

```
gpg --keyserver keyserver.ubuntu.com --recv "D681 1ED3 ADEE B844 1AF5 AA8F 4528 B6CD 9E61 EF26"
```

Since I'm installing in a proxy-only environment this fails with

```
gpg: keyserver receive failed: Connection timed out
```

The problem is that the proxy is not passed over to gpg like

```
gpg --keyserver keyserver.ubuntu.com --keyserver-options http-proxy="http://proxy.domain.de:3128" --recv "D681 1ED3 ADEE B844 1AF5 AA8F 4528 B6CD 9E61 EF26"
```

in gpg.py. Therefor install fails in curtin command apt-config.