fix: keep go caches on disk instead of tmpfs /tmp

On the target host /tmp is a tmpfs sized at half of RAM (~970M), so the
go module cache and build cache together filled it and every module
download failed with "no space left on device". Move both caches to
/var/tmp, which lives on the root filesystem, and create both cache
directories up front rather than only the build cache.

common.mk

@@ -2,8 +2,8 @@ GO_ARGS ?=
 GO_PLUGIN_MAKE_TARGET ?= build
 GO_REPO_ROOT := /go/src/github.com/dokku/dokku
 BUILD_IMAGE := hub.diyla.com/golang:1.27.3
-GO_BUILD_CACHE ?= /tmp/dokku-go-build-cache
-GO_MOD_CACHE   ?= /tmp/dokku-go-mod-mod
+GO_BUILD_CACHE ?= /var/tmp/dokku-go-build-cache
+GO_MOD_CACHE   ?= /var/tmp/dokku-go-mod-mod
 GO_ROOT_MOUNT  ?= $$PWD/../..:$(GO_REPO_ROOT)
 ifndef TARGETARCH
 TARGETARCH     := $(shell dpkg --print-architecture 2>/dev/null || uname -m | sed -e 's/x86_64/amd64/' -e 's/aarch64/arm64/')
@@ -15,7 +15,7 @@ GOARCH         ?= $(TARGETARCH)
 build: $(BUILD)
 
 build-in-docker: clean
-	mkdir -p /tmp/dokku-go-build-cache
+	mkdir -p $(GO_BUILD_CACHE) $(GO_MOD_CACHE)
 	docker run --rm \
 		-v $(GO_ROOT_MOUNT) \
 		-v $(GO_BUILD_CACHE):/root/.cache \