Makefile: improvements (#2374)

- Add NIXPKGS_REV, so we can pass arbitrary revisions for testing (for
  example, `release-21.05` so we can test backports).
- Add format target, that calls the format script.

Makefile

@@ -1,5 +1,6 @@
-.PHONY: all all-tests test
-NIX_PATH := nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz
+.PHONY: all all-tests test format
+NIXPKGS_REV := nixpkgs-unstable
+NIX_PATH := nixpkgs=https://github.com/NixOS/nixpkgs/archive/${NIXPKGS_REV}.tar.gz
 
 all: all-tests
 
@@ -11,3 +12,6 @@ ifndef TEST
 	$(error Use 'make test TEST=<test_name>' to run desired test)
 endif
 	nix-shell --pure tests -I ${NIX_PATH} -A run.${TEST}
+
+format:
+	./format