env/freebsd-amd64: add FreeBSD 11.3 image
env/freebsd-amd64: add FreeBSD 11.3 image

Change-Id: I7895242a05bd7d642bf33250212cd5e3e717254c
Reviewed-on: https://go-review.googlesource.com/c/build/+/186038
Run-TryBot: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/build,golang/build,golang/build,golang/build