[keyserver] Add bash/source-nvm.sh to yarn babel-build

Summary:
We have a step in the dev_instructions where the developer is supposed to run:

yarn babel-build
yarn rsync
yarn script dist/scripts/create-db.js

in the keyserver directory.

However, unlike when we run yarn dev in keyserver, we don't source node from nvm. That means that if the developer has an incompatible version of node installed, running yarn script dist/scripts/create-db.js may fail.

(In the future this sort of thing will be handled by nix)

Test Plan: Ran yarn babel-build after change and based on the logs (https://gist.github.com/atulsmadhugiri/208338778e507ae59001e63aef75631a), it sourced node 16.9.0 from nvm and completed successfully.

Reviewers: palys-swm, benschac, dereknelson, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3995