release-cross.nix: test cross compilation to x86_64-freebsd

PR #138711 added NetBSD. FreeBSD has come a long ways since the last
release. It would be good to have this cached for 23.05.

+1
+1
pkgs/top-level/release-cross.nix
··· 227 227 riscv32-embedded = mapTestOnCross lib.systems.examples.riscv32-embedded embedded; 228 228 rx-embedded = mapTestOnCross lib.systems.examples.rx-embedded embedded; 229 229 230 + x86_64-freebsd = mapTestOnCross lib.systems.examples.x86_64-freebsd common; 230 231 x86_64-netbsd = mapTestOnCross lib.systems.examples.x86_64-netbsd common; 231 232 232 233 # we test `embedded` instead of `linuxCommon` because very few packages