build: Enable building binary only on macos (#918)

Current config only allows building local binaries on Linux machines so
adding config to allow building binaries on macos without changing the
release config (and hence still only releasing linux binaries as
required).

Author: jimmidyson

make/goreleaser.mk

@@ -16,6 +16,7 @@ build-snapshot: go-generate ; $(info $(M) building snapshot $*)
 		--snapshot \
 		--clean \
 		--parallelism=$(GORELEASER_PARALLELISM) \
+		--config=<(env GOOS=$(shell go env GOOS) gojq --yaml-input --yaml-output '.builds[0].goos |= (. + [env.GOOS] | unique)' .goreleaser.yml) \
 		$(if $(BUILD_ALL),,--single-target)
 
 .PHONY: release