Make the binary the default Make target

Plain make now builds sfdupes (previously the default was all =
check + build); make build remains as an alias, so the Dockerfile
and existing habits keep working. The sfdupes target is phony: go
build's own cache decides what to recompile.

README.md

@@ -393,7 +393,8 @@ Additional requirements:
 
 The `Makefile` is the single source of truth for all operations:
 
--   `make build` — build the `sfdupes` binary (cgo disabled).
+-   `make` / `make build` — build the `sfdupes` binary (cgo
+    disabled); building is the default target.
 -   `make test` — run the test suite (30-second timeout; reruns with
     `-v` on failure).
 -   `make lint` — run `golangci-lint` with the repo config.