use SUDO_USER if "logname" does not return any result

This can happen in certain situations e.g. when you enter an
OpenVZ container via "vzctl enter CTID" the login name is not
available.

scripts/build.py

@@ -402,7 +402,7 @@ ARCH = ['i386']
 
 def check_setup_schroot(config):
     check_running_on_debian()
-    login = get_output('logname')
+    login = get_output('logname') or os.environ.get('SUDO_USER')
     if not login:
         error('Unable to determine the login for which schroot access is to be given.')
 
@@ -412,7 +412,7 @@ def check_setup_schroot(config):
 def build_setup_schroot(config, basedir):
     install_packages('git', 'debootstrap', 'schroot', 'rinse')
 
-    login  = get_output('logname')
+    login  = get_output('logname') or os.environ.get('SUDO_USER')
     chroot = config[1+config.rindex('-'):]
     for arch in ARCH:
         print '******************* %s-%s' % (chroot, arch)