CI: Cirrus: bump used FreeBSD from 12.1 to 13.0

CI freebsd_12 job currently fails to build PRs, because of:
```
ld-elf.so.1: /usr/local/bin/bison: Undefined symbol "fread_unlocked@FBSD_1.6"
```

According to FreeBSD issue tracker[1], the proper solution is to upgrade to a
supported release, so do that for our CI.

[1]: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253452

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
1 file changed