leaseshelper: fix crash
    
    Commit baafe668 introduced new leaseshelper with a crash of freeing
    env string. Calling 'getenv()' inside 'virGetEnvAllowSUID()' may
    return a static string and we definitely should not free it.
    
    The author probably want to free the copy of that string.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>