[cbuild] Remove the option to specify binary location using environment variable $CMSIS_BUILD_ROOT

[jkrech]

When the shell script cbuild.sh was used to orchestrate the pack installation, cbuildgen, cmake and ninja invocation the environment $CMSIS_BUILD_ROOT was introduced. Since the Golang implementation of cbuild this environment is no longer needed but is still considered but not part of the setup.
Users of CMSIS-Toolbox must ensure that $CMSIS_BUILD_ROOT is not set to avoid that cbuild invokes potentially outdated binaries installed in other locations.

How to fix: cbuild shall no longer read/follow the $CMSIS_BUILD_ROOT environment variable but invoke tools from the same directory it is located in (side by side).