Fix building under NODE_ENV=production

This is meant to only control webpack. But `npm install` skips
installing `devDependencies` under `NODE_ENV=production`, which breaks
the package build. So always unset it for npm.

This fixes `NODE_ENV=production make` in a clean tree.

Makefile

@@ -150,7 +150,8 @@ test/common:
 $(NODE_MODULES_TEST): package.json
 	# if it exists already, npm install won't update it; force that so that we always get up-to-date packages
 	rm -f package-lock.json
-	npm install
-	npm prune
+	# unset NODE_ENV, skips devDependencies otherwise
+	env -u NODE_ENV npm install
+	env -u NODE_ENV npm prune
 
 .PHONY: all clean install devel-install dist-gzip srpm rpm check vm update-po