avoid using HTTP_PROXY environment variable

People run into problems on upgrades, for example:

 export http_proxy=http://172.xxx.xxx.xxx:8888/
 aptitude update && aptitude full-upgrade

This restarts pveproxy, and after that login was no longer possible.

Another way to reproduce the bug is:

 http_proxy=http://1.2.3.4:8888/ pveproxy --debug

PVE/HTTPServer.pm

@@ -326,6 +326,7 @@ sub proxy_request {
 	    headers => $headers,
 	    timeout => 30,
 	    recurse => 0,
+	    proxy => undef, # avoid use of $ENV{HTTP_PROXY}
 	    keepalive => $keep_alive,
 	    body => $content,
 	    tls_ctx => $self->{tls_ctx},