Add Makefile go.mod.cachedir for release pipeline compatibility (#279)

We only had go.cachedir, provider-kubernetes also had go.mod.cachedir.
This is used here:

https://github.com/crossplane-contrib/provider-workflows/blob/e2da965b49f5bfff36da597dce4737cc30c83c7b/.github/workflows/publish-provider.yml#L110-L114

Author: chlunde

Makefile

@@ -103,6 +103,9 @@ submodules:
 go.cachedir:
 	@go env GOCACHE
 
+go.mod.cachedir:
+	@go env GOMODCACHE
+
 # NOTE(hasheddan): we must ensure up is installed in tool cache prior to build
 # as including the k8s_tools machinery prior to the xpkg machinery sets UP to
 # point to tool cache.
@@ -131,7 +134,7 @@ dev-clean: $(KIND) $(KUBECTL)
 	@$(INFO) Deleting kind cluster
 	@$(KIND) delete cluster --name=$(PROJECT_NAME)-dev
 
-.PHONY: submodules fallthrough test-integration run crds.clean dev dev-clean
+.PHONY: submodules fallthrough test-integration run crds.clean dev dev-clean go.mod.cachedir go.cachedir
 
 # ====================================================================================
 # Special Targets