fix #4815: pmgsh: fix calling the api paths directly

fix #4815: pmgsh: fix calling the api paths directly

if we get a command directly, we don't initialize the $rpcenv
variable anymore.

To fix it, make it a local variable of the pmg_command function.
We now make one extra '->get()' call per command (as opposed to
once per program), but that shouldn't cost us anything really.

Reported in the forum: https://forum.proxmox.com/threads/.130008/

Fixes: 1583283 ("pmgsh: initialize RPC/REST environment late")
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
 [S.I: add bugreference, fixes trailer and shorten forum link]
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>