Add Python 3.9 to pre-cached image

[davidism]

Describe the bug

Python 3.9 is not pre-cached, so it must be installed, which takes about 60 seconds on Windows and Mac. People made a bunch of PRs saying 3.9 is supported now, and I only found out it's only "sort of" supported after I merged them and discovered the slowdown. Had to revert as it was too frustrating to wait 10x longer for Windows and Mac than all the rest of the envs when I was reviewing/iterating on PRs.

Which version of the action are you using?

• v1
• v2
• Some other tag (such as v2.0.1 or master)

Environment

• self-hosted
• Linux
• Windows
• Mac

Python Versions

3.9

To Reproduce

1. Specify version 3.9
2. Wait for it to install

[davidism]

Subscribe to actions/runner-images#1740 instead.