[shac] Make default clang-format tool parameter a function call.

In fuchsia.git, shac checks need access to the 'ctx' variable
in order to determine the location of the prebuilt tools.

This changes the default tool parameter from a string to
a lambda, so this check can be used in both environments.

Tested by pinning this change in fuchsia.git and running the tools.

Change-Id: I641616402d0cc8ae0aa161adc74bd36ceef1643c
Reviewed-on: https://fuchsia-review.googlesource.com/c/shac-project/checks-cpp/+/1040134
Reviewed-by: Darren Chan <chandarren@google.com>