Runners set up environment variables corresponding to the runner
context's properties. The runner context documents the runner.os,
runner.temp, and runner.tool_cache properties, so document these
three as supported environment variables similarly. The tool_cache
property value is not documented as being used for GitHub AE (see
context-and-expression-syntax-for-github-actions.md), so just leave
$RUNNER_TOOL_CACHE undocumented in this case.