build: enable journal in promtail linux release build (grafana#1072)

* build: override promtail release build with cgo-enabled binary

The published binary of promtail for linux/amd64 was not built with CGO
support, so journal tailing could not be used.

This is kind of a dirty hack: we just re-build promtail at the end of
building everything else with CGO enabled. It's not clear how it could
be done more cleanly.

Fixes grafana#1057.

* build: break out gox builds

Makefile

@@ -193,10 +193,13 @@ cmd/promtail/promtail-debug: $(APP_GO_FILES) pkg/promtail/server/ui/assets_vfsda
 # Releasing #
 #############
 # concurrency is limited to 4 to prevent CircleCI from OOMing. Sorry
-GOX = gox $(GO_FLAGS) -parallel=4 -output="dist/{{.Dir}}-{{.OS}}-{{.Arch}}" -arch="amd64 arm64 arm" -os="linux"
+GOX = gox $(GO_FLAGS) -parallel=4 -output="dist/{{.Dir}}-{{.OS}}-{{.Arch}}"
+CGO_GOX = gox $(DYN_GO_FLAGS) -cgo -parallel=4 -output="dist/{{.Dir}}-{{.OS}}-{{.Arch}}"
 dist: clean
-	CGO_ENABLED=0 $(GOX) -osarch="windows/amd64" ./cmd/loki
-	CGO_ENABLED=0 $(GOX) -osarch="darwin/amd64 windows/amd64 freebsd/amd64" ./cmd/promtail ./cmd/logcli
+	CGO_ENABLED=0 $(GOX) -osarch="linux/amd64 linux/arm64 linux/arm windows/amd64" ./cmd/loki
+	CGO_ENABLED=0 $(GOX) -osarch="linux/amd64 linux/arm64 linux/arm darwin/amd64 windows/amd64 freebsd/amd64" ./cmd/logcli
+	CGO_ENABLED=0 $(GOX) -osarch="linux/arm64 linux/arm darwin/amd64 windows/amd64 freebsd/amd64" ./cmd/promtail
+	CGO_ENABLED=1 $(CGO_GOX) -osarch="linux/amd64" ./cmd/promtail
 	gzip dist/*
 	pushd dist && sha256sum * > SHA256SUMS && popd