[mk-sbuild] syntax error in proxy configuration causes aptdaemon to fail to build

One of my sbuild instances failed to build aptdaemon as follows:

ERROR: test_ (tests.test_configparser.ConfigurationParserTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/«PKGBUILDDIR»/tests/test_configparser.py", line 75, in test_
    config = self.parser.parse(fd.readlines())
  File "/«PKGBUILDDIR»/aptdaemon/config.py", line 161, in parse
    (char, line))
ValueError: Unknown char '#' in line: '## proxy settings copied from mk-sbuild
'

This is due to incorrect comment syntax. APT configuration files use C/C++-style comments (// or /* ... */), not leading-#.