sources/curl: use --user-agent option to set the user-agent

Setting the user-agent using `--header` is broken in combination with
`--location`, `--proxy`, and an https endpoint which redirects. The
user-agent sent to the proxy changes after the client is redirected,
tripping up proxies.

For more information see https://issues.redhat.com/browse/RHEL-45364

sources/test/test_curl_source.py

@@ -222,6 +222,6 @@ def test_curl_user_agent(mocked_run, tmp_path, sources_service):
 
     for call_args in mocked_run.call_args_list:
         args, _kwargs = call_args
-        idx = args[0].index("--header")
-        assert "User-Agent: osbuild" in args[0][idx + 1]
+        idx = args[0].index("--user-agent")
+        assert "osbuild" in args[0][idx + 1]
         assert "https://osbuild.org/" in args[0][idx + 1]