Change CLI source to point to @saleor/cli in npm (#2252)

CLI is migrated to new namespace. 
Also, `@next` shouldn't be used on production code

.github/workflows/tests-nightly.yml

@@ -50,7 +50,7 @@ jobs:
 
       - name: Install saleor cli
         id: install-saleor-cli
-        run: npm i -g saleor-cli@next
+        run: npm i -g @saleor/cli
 
       - name: Cache node modules
         uses: actions/cache@v2