Improve RUNNER_TOOL_CACHE default setting

[ikari7789]

When using ${{ runner.tool_cache }} in the workflow on a self-hosted runner, it returns the value of several different environment variables in coalescing order.

https://github.com/actions/runner/blob/591f8c351058ed1251ecf04a4195ff83a6828e12/src/Runner.Common/HostContext.cs#L263

Currently it seems that the scripts are hard-coding this to /tmp which ends up storing the problem matchers in a different directory potentially from the runner's environment when accessing via ${{ runner.tool_cache }}.

setup-php/src/scripts/unix.sh

Line 154 in 9eaa66d

add_env RUNNER_TOOL_CACHE /tmp

Proposing to set this variable in a similar fashion by checking if existing environment variables are available.