[rust-gdb] relax the GDB version regex

[rust-gdb] relax the GDB version regex

The pretty-printer script is checking `gdb.VERSION` to see if it's at
least 8.1 for some features. With `re.match`, it will only find the
version at the beginning of that string, but in Fedora the string is
something like "Fedora 8.2-5.fc29". Using `re.search` instead will find
the first location that matches anywhere, so it will find my 8.2.