This fix updates the build docs to include the path of `build/` for
shell scripts. The reason is that while trying to follow the `build/README.md`,
it is not obvious that all the scripts, e.g., `run.sh make`, `shell.sh`,
etc. needs to be executed from the root directory (vs. executed from the
`build/` directory). In other words,
the executation should be:
```
build/run.sh make
build/make-clean.sh
...
```

This fix adds `build/` so that it is easy for user to follow the steps.