.builds: upgrade FreeBSD builder

It's currently failing, and upgrading it may fix it.

While here, track lates FreeBSD to avoid upgrade toil.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

.builds/freebsd.yml

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: Unlicense OR MIT
-image: freebsd/13.x
+image: freebsd/latest
 packages:
  - libX11
  - libxkbcommon