community/go: set -buildmode=pie using GOFLAGS
This fixes the cannot find runtime/cgo
-warning currently output occasionally by go itself. Additionally, it allows enabling more parts of the test suite in the future.
See also: https://lists.alpinelinux.org/~alpine/devel/%3C20191129221308.hy6ybayxu2nsu5dy%40wolfsden.cz%3E
Edited by Sören Tempel