The bootstrap script now begins with verifying
that all required tools are on the PATH, and fails
otherwise.

Previously, the bootstrap script could have spent
minutes working before it tried to use a missing
tool, only then reporting the absence.

This PR alleviates https://github.com/bazelbuild/bazel/issues/7641

Change-Id: I4c090113fe486f9dff9445adc703c76043088095

Closes #7991.

Change-Id: I5b9e3fbd8ae46fe2b74983305521b6385d82bc56
PiperOrigin-RevId: 243047535