Add auto-merge for `nodenv-bot` PRs

[ai]

On new Node.js version release nodenv-bot bot sends PR like #736

It could take a few days to merge it.

It will be very nice to add CI script to merge green PRs from nodenv-bot automatically.

[ai]

I can ask junior developers to create PR for that if you want

[usmanyunusov]

PR - #740

[jasonkarns]

The main concern I have with auto-merging is that node-build has a high degree of power on user's machines. Each build definition is essentially a bash script being executed.

Before we have any automatic merging, we should have additional safeguards in place (probably extra CI suites), that ensure each build definition is only invoking our functions and is only fetching from expected sites (both could be checked against allowlists). If we get this additional CI check in place, such that it is optional on PRs (shouldn't prevent manual PR merges), yet required for auto-merging, then I would be more inclined to land an automerge setup.

[col3name]

PR - #756