fix: respect proxy env vars in anonymous http client

The anonymous HTTP client was using a bare `http.Transport{}`
which does not respect `HTTP_PROXY`/`HTTPS_PROXY` environment
variables. This change clones `http.DefaultTransport` instead,
which has `Proxy: http.ProxyFromEnvironment` set by default.

Related to #1509

Signed-off-by: Diogo Correia <diogo.correia@siemens.com>

Author: dvcorreia

github/config.go

@@ -87,7 +87,7 @@ func (c *Config) Anonymous() bool {
 }
 
 func (c *Config) AnonymousHTTPClient() *http.Client {
-	client := &http.Client{Transport: &http.Transport{}}
+	client := &http.Client{Transport: http.DefaultTransport.(*http.Transport).Clone()}
 	return RateLimitedHTTPClient(client, c.WriteDelay, c.ReadDelay, c.RetryDelay, c.ParallelRequests, c.RetryableErrors, c.MaxRetries)
 }