Update FreeBSD build

[devnexen]

Actually a docker build to compile freebsd component binaries is used, for the 12.x versions. Problem is when we want to update in order to build for a more recent version, we fall into a conundrum. Ideally , instead, using Cirrus to build FreeBSD binaries would be more straightforward or how do we update the toolchain used to build rustc ?

[jieyouxu]

@devnexen you may want to get in touch with T-infra on zulip (https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra) to discuss this.

[devnexen]

Problem solved with this commit.