chore: switch npm client to pnpm in lerna config

Author: vmasek

lerna.json

@@ -1,4 +1,5 @@
 {
+  "npmClient": "pnpm",
   "version": "6.4.0",
   "command": {
     "publish": {
@@ -16,4 +17,4 @@
     "**/*.spec.tsx",
     "**/package-lock.json"
   ]
-}
+}