Github actions: update to Node 16

[tripleee]

Recent nightly jobs have this notice:

Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions/checkout, actions/checkout

[tripleee]

Also, test-pushed gets this:

The set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

It's not using set-output directly so I'm guessing it's related to the above.

[tripleee]

The documentation in the link is pretty nondescript; but I upgraded to v3 of actions/checkout which according to their README should get me to Node v16.

[tripleee]

The fix was insufficient; I still see the warning banner at the bottom of the build's page.

[tripleee]

Feedback at https://github.com/orgs/community/discussions/39151

[tripleee]

actions/checkout#959 alleges that upgrading to v3 should fix this. Maybe let's just wait and check again.

[tripleee]

Actually, the warning is gone from the nightly job now. I guess something is still left in the push test to review. It also employs actions/setup-python@v2; there's a v4 available - let's try that.

[tripleee]

Yay, the banner is gone!