fix(ci): don't capture tool-download noise as a formatting diff

The gofumpt check merged stderr into the diff variable, so 'go:
downloading ...' lines made it always fail. Move tool installation into
its own step (adding GOPATH/bin to PATH) and capture only stdout.

Makefile

@@ -58,7 +58,7 @@ fmt: ## Format code (gofumpt + goimports via golangci-lint)
 
 fmt-check: ## Check formatting (gofumpt + goimports via golangci-lint)
 	@echo "Checking formatting..."
-	@diff=$$($(GOLANGCI_LINT) fmt --diff --config=.golangci.json 2>&1); \
+	@diff=$$($(GOLANGCI_LINT) fmt --diff --config=.golangci.json); \
 	if [ -n "$$diff" ]; then echo "$$diff"; echo "Run: make fmt"; exit 1; fi
 
 staticcheck: ## Run staticcheck