flake: run tests in flake check, actually check fourmolu

Makefile

@@ -1,4 +1,5 @@
-SHELL := /usr/bin/env bash
+# This really ought to be `/usr/bin/env bash`, but nix flakes don't like that.
+SHELL := /bin/sh
 
 .PHONY: hoogle format haddock usage
 
@@ -10,7 +11,7 @@ usage:
 	@echo "  format -- Format the project"
 	@echo "  haddock -- Generate Haddock docs for project"
 
-hoogle: 
+hoogle:
 	hoogle generate --local=haddock --database=hoo/local.hoo
 	hoogle server --local -p 8081 >> /dev/null &
 	hoogle server --local --database=hoo/local.hoo -p 8082 >> /dev/null &
@@ -21,5 +22,8 @@ format:
 	git ls-tree -r HEAD --full-tree --name-only | grep -E '.*\.nix' | xargs nixfmt
 	git ls-tree -r HEAD --full-tree --name-only | grep -E '.*\.cabal' | xargs cabal-fmt -i
 
+format_check:
+	find -name '*.hs' -not -path './dist-*/*' | xargs fourmolu $(FORMAT_EXTENSIONS) -m check
+
 haddock:
 	cabal haddock --haddock-html --haddock-hoogle --builddir=haddock