Move the directory list into a variable

GNUmakefile

@@ -1000,15 +1000,15 @@ endif
 tools:
 	cd internal/tools && go generate -tags tools ./
 
-LINTFILESCMD=find src/os/ src/reflect/ -type f -name '*.go'
+LINTDIRS=src/os/ src/reflect/
 .PHONY: lint
 lint: tools ## Lint source tree
 	revive -version
 	# TODO: lint more directories!
 	# revive.toml isn't flexible enough to filter out just one kind of error from a checker, so do it with grep here.
 	# Can't use grep with friendly formatter.  Plain output isn't too bad, though.
 	# Use 'grep .' to get rid of stray blank line
-	revive -config revive.toml compiler/... $$( $(LINTFILESCMD) ) \
+	revive -config revive.toml compiler/... $$( find $(LINTDIRS) -type f -name '*.go' ) \
 		| grep -v "should have comment or be unexported" \
 		| grep '.' \
 		| awk '{print}; END {exit NR>0}'