refactor(mk): move build rules into separate scripts (#855)

See https://github.com/ooni/probe/issues/2218

CLI/go-build-alpine

@@ -0,0 +1,14 @@
+#!/bin/sh
+set -euxo pipefail
+apk update
+apk upgrade
+apk add --no-progress gcc git linux-headers musl-dev
+# some of the following exports are redundant but are however
+# useful because they provide explicit logging
+export CGO_ENABLED=1
+export GOARM=$GOARM
+export GOOS=$GOOS
+export GOARCH=$GOARCH
+go build -o ./CLI/$PRODUCT-$GOOS-$OONIARCH \
+	-ldflags='-s -w -extldflags "-static"' \
+	$GOLANG_EXTRA_FLAGS $PACKAGE