chore: reduce concurrency to avoid bootstrap issues due to npm locks

It happens very frequently for me with `npm run update-packages`, which
is also part of our release process.

Sample error:

lerna ERR! npm install exited 254 in '@loopback/service-proxy'
lerna ERR! npm install stderr:
npm ERR! code ENOENT
npm ERR! syscall lchown
npm ERR! path /Users/rfeng/.npm/_locks/staging-1f9c9b6dc2a3f888.lock
npm ERR! errno -2
npm ERR! enoent ENOENT: no such file or directory, lchown '/Users/rfeng/.npm/_locks/staging-1f9c9b6dc2a3f888.lock'
npm ERR! enoent This is related to npm not being able to find a file.
npm ERR! enoent

lerna.json

@@ -11,7 +11,7 @@
   ],
   "command": {
     "bootstrap": {
-      "concurrency": 8,
+      "concurrency": 4,
       "forceLocal": true
     },
     "version": {