giscanner: use re.match() instead of re.search()

giscanner: use re.match() instead of re.search()

From Python's documentation:
Python offers two different primitive operations based on
regular expressions: match checks for a match only at the
beginning of the string, while search checks for a match
anywhere in the string (this is what Perl does by default).

The different behavior between match() and search() doesn't
matter in our case and conceptually we are checking if a
piece of text matches a certain pattern, so match() is a
better fit.

https://bugzilla.gnome.org/show_bug.cgi?id=688897