Add support for getting a temporary directory

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)

Start by reading packages/cache/src/internal/cacheUtils.ts and packages/tool-cache/src/tool-cache.ts, which already reference temporary-directory handling, then inspect the RUNNER_TEMP environment entry point. Determine whether @actions/io or @actions/core is the appropriate home; done means callers can obtain the runner temporary directory and documentation covers cleanup after jobs and runner crashes.