Instead, search for stuff up the directory tree, with the main
functionality being to look for `Gulpfile.js` and assume the resulting
directory is the root.
(Unfortunatley, this is implemented twice, one in `scripts` and another
in `src`. It's not possible to use a single implementation for both
since that would require assuming a directory structure which this is
intended to avoid.)
Also, in `scripts/build/projects.js`, abstracdt common exec
functionality into a local helper, and use full paths based on the above
search instead of assuming relative paths assuming CWD being in the
project root.
