Fix format.py

Fix format.py

In format.py, we were downloading bootstrap.sh/cmd to the current working
directory, but assuming that it was in coreclr/bootstrap, which is
incorrect. This change downloads it directly to coreclr/bootstrap, so that
we download and run to the same location.

This change also modifies where we install the newer version of dotnetcli
needed for jitutils to the coreclr/dotnetcli-jitutils directory. This
change is necessary because build.cmd->run.cmd->init-tools.cmd removes the
Tools directory, including our dotnetcli directory, so we don't want our
version of the cli to be trashed.

We also clean up our work at the end of the script by removing the
created jitutils and dotnetcli directories, and the bootstrap script.

Finally, this change adds some comments to tell the user what it's doing.

Commit migrated from https://github.com/dotnet/coreclr/commit/4d51ced0cd78abb9439f31d6599600b33d7e8e0d