Allow overriding NODE_DEFAULT_VERSION @ automatic build

**Is your feature request related to a problem? Please describe.**
The automatic build uses nvm to install node and then additionally selected packages such as yarn and amplify cli. But the version is still hardcoded to 12.

As a workaround I manually install node 14 but then the additional packages are not available, so have to be manually installed as well. Every customer using automatic build with recent versions has to experience this issue..

**Describe the solution you'd like**
It should be possible to simply set NODE_DEFAULT_VERSION to anything you want, or simply set it as an additional package in the console. Same for python and whatever else.

Start by tracing the automatic build flow that uses nvm and installs Node.js, Yarn, and Amplify CLI, then inspect how NODE_DEFAULT_VERSION is handled. Confirm the desired behavior by checking whether a configured Node.js version also receives the selected additional packages, including the analogous Python case.

Written by the indexing model from the issue text.