Add support for getting a temporary directory

[konradpabjan]

The runner has a temporary directory that gets cleaned up after each job. You can access it by doing process.env['RUNNER_TEMP'], this can be used for a lot of things such as creating temporary files for tests or during a job and users don't need to worry about this being cleaned up.

A lot of first party actions and third party actions could use this. A lot of first party actions are in-fact using the temp directory currently:

• cache
• tool-cache
• setup-java and setup-python will soon be using the temp directory too for a variety of things

A method should be added to either @actions/io or @actions/core that will allow users to quickly get a temporary directory.

There should also be documentation added that mentions when this gets cleaned up, and under what circumstances it won't (runner crashes)

[bbqsrc]

This is reasonably important for sensitive things such as temporary secrets on self-hosted runners on macOS or Windows where containers are not possible.

[koppor]

For the googlers coming here: In the case of a Docker-based action, one CANNOT access that. See https://github.community/t/how-can-i-access-the-current-repo-context-and-files-from-a-docker-container-action/17711/8 for details.

[minherz]

What about using the runner's temporary directory runner.temp?

[nelsonic]

runner.temp doesn't appear to be writeable.
is there a usage example that demonstrates creating & deleting files in the runner.temp directory?