Support env setting of AGENT_TOOLSDIRECTORY as in the setup-python action

Description:
as in setup python

  if (IS_MAC) {
    process.env['AGENT_TOOLSDIRECTORY'] = '/Users/runner/hostedtoolcache';
  }

  if (process.env.AGENT_TOOLSDIRECTORY?.trim()) {
    process.env['RUNNER_TOOL_CACHE'] = process.env['AGENT_TOOLSDIRECTORY'];
  }

  core.debug(
    `Python is expected to be installed into ${process.env['RUNNER_TOOL_CACHE']}`
  );

setup-node should read the AGENT_TOOLSDIRECTORY environment variable and if it exists, sets the RUNNER_TOOL_CACHE variable equal to it

A GitHub support agent said:

I was able to get this to work by forking actions/setup-node and copying the same implementation on actions/setup-python to allow support for AGENT_TOOLSDIRECTORY on my fork - essentially just throwing in those few lines where the run function is defined here.

Justification:
Other actions already support this declaration and we need to use it to populate a caching system we are building to avoid hitting GitHub.com so often to install programming languages.

Are you willing to submit a PR?
possibly, I will need to check schedule with our engineering team.

Start in src/main.ts and compare its run entry point with the linked setup-python implementation. Done means a non-empty AGENT_TOOLSDIRECTORY value is recognized so that RUNNER_TOOL_CACHE uses the same directory, with the behavior checked against the repository's existing validation.