Missing support for freebsd/amd64

[amiramix]

This target is missing in install.sh and in releases. golang 1.12 is available on FreeBSD and the tooling is similar to Darwin so it should be easy to add it unless Docker is a strict requirement.

I can do the porting and test on FreeBSD but would need some pointers how to go about adding the target.

[tpounds]

@amiramix FreeBSD support was added in #613 and is now available since release v1.18.0.