chore: fix Makefile

Author: mrexox

Makefile

@@ -7,7 +7,7 @@ build-with-coverage:
 	go build -cover -ldflags "-s -w -X github.com/evilmartians/lefthook/internal/version.commit=$(COMMIT_HASH)" -o lefthook
 
 install: build
-	cp lefthook $(GOPATH)/bin/
+	cp lefthook $$(go env GOPATH)/bin
 
 test:
 	go test -cpu 24 -race -count=1 -timeout=30s ./...