ci: install gh cli in container so that prepare step doesn't fail

Author: NlightNFotis

.github/workflows/__test-proxy.yml

@@ -1,10 +1,13 @@
 name: "Proxy test"
 description: "Tests using a proxy specified by the https_proxy environment variable"
-versions: ["linked", "stable"]
+versions: ["linked", "nightly-latest"]
 operatingSystems: ["ubuntu"]
 container:
   image: ubuntu:22.04
   options: --dns 127.0.0.1
+  run: |
+    apt-get update
+    apt-get install -y gh
 services:
   squid-proxy:
     image: ubuntu/squid:latest